I like the tile on the floor and shower. The white shower curtain looks a little plain though.
I'm having a hard time seeing the actual colors of the tile, but maybe a chocolate brown shower curtain? Needs some drama.... Also, how about something over the toilet. Maybe a cabinet or floating shelf? Looks bare there.

Replace the mirror over the sink with a new framed mirror with a dark wood frame to match the cabinets. Or you could hang 3 smaller framed mirrors....one centered over the sink.......

Change knobs and add rugs.

Well obviously a little color on the wall would help, I would also repaint the ceiling to something a little softer like benjamin moore cloud white not in a pearl type of finish, probably in an ultimatt type of finish. and an earth tone color on the walls something light in color also in ultimatte finish. Adding a wood picture frame type molding around the mirror in the same finish as the mirror would help. Swap out the floor register to something that doesnt stick out like a sore thumb would be good. A few floating shelves like the plynnehome was a great idea. Some scented candles and small glass bowl with stones to give it that spa atmospere. On the big plain wall, add some artwork. Also where do you keep your towels

I would paint the walls a darker shade. The pictures are difficult to tell your lighting so I won't give you a color number but it should be a darker shade (in the beige family) than what you have. Replace the shower bar to a curved rod. Then place it as close to the ceiling as possible. You will need to find an extra long curtain. (Bed Bath and Beyond has a nice selection online). I make my own from sheets if you are handy. It should add color. I would suggest a stripe pattern containing a bold burnt orange. Add a thick, thirsty rug to the floor. Towels are a necessity. Add thirsty white towels and wrap a coordinating ribbon around them. Place on the corner of the vanity. Select brown glass containers to place on the vanity. Lastly I would frame the mirror. There are several online resources. Also you can take your measurements and go to Home Depot. They will help you find a trim and they will cut it for you. You can apply it directly to the mirror with double sided tape or Velcro. Good luck!

It's really bare right now, and there aren't any towel bars (I ripped the ugly builder one out last week and just finished patching the drywall).

I definitely plan to paint. Currently considering Elmira white (Ben Moore) for the walls.

I don't think I'm up to removing that big mirror, but am definitely thinking of framing it out.

Not certain of where to place replacement towel bars/rings. Any ideas?

I agree to paint the walls a darker shade. Framing out the mirror with molding and paint/stain to match the vanity color would look nice. How about installing a hotel style towel holder to give it a "spa" like feel? Notice how the below photo with the towel holder and framed large mirror. Instead of a towel bar on the wall, robe hooks would also give it a "spa" look.
https://www.houzz.com/photos/2011-hhl-bathrooms-contemporary-bathroom-phvw-vp~136048

I know it's a bit tough to tell the colours right now. The walls are currently just barely off white because it was the whitest of the 12 white choices our builder gave us, haha!

The counter is a yellow/brown and the tiles are more light brown/grey.

The light fixture is getting changed out to one with white covers and pure white bulbs (not yellow, not blue).

If you can spring for a new faucet, a more modern looking faucet will give the space a fresher look instantly.

Hooks instead of towel bars?

I should probably add that the photo of the big blank wall is to show that there is a big blank wall. The TP holder used to be there and I'm not sure whether to put the new one back on the wall or on the side of the cabinet closest to the toilet.

The light fixture in the photo is the one I'm getting rid of. There are 2 different bulbs in it right now, because I needed extra light while repairing the drywall. Builder glued the old fixtures to the wall an I took a nasty chunk out when I removed the TP holder (see photo!). Glad it's all fixed now!

Would love to change out the faucet, but it's not in the budget right now. :(

Several things could be done. You can replace mirror with one that has a frame. You can put a crown molding in and paint ceiling darker gray. Use an 84" patterned curtain for height and color. Bed, Bath and Beyond sells an 84" clear liner. You would use a double bar. Paint a soft gray to the walls. Add art to the walls and a unique cabinet on opposite wall of vanity, if it fits. A nice lux rug in front of vanity will feel soft under foot.

Most of the house will be gray (stonington gray HC-170 Benjamin Moore), so I'd like to use a different colour in the bathroom.

I picked up some paint chips in beige... The same chips are in both photos, but different orders. What do you think looks best?

Also, this is the vanity light I got to replace the one that currently has 2 different bulbs. ;)

The shades are much whiter than they look on the box.

You have no window in the room. If so buy drapes to match shower curtain. Use adhesive to add a frame right to the edge of your mirror. Crown & ceiling paint are both good ideas. Get an individual toilet paper holder that can sit in the floor. Use LED bulbs if possible for your light fixture. Last 20,000 hrs, cooler light & economical. Add a large silk orchid to place on the counter. Make sure you have plenty of towel rods or hooks. Look at Shop on Houzz in Bathrooms for ideas. Towels, curtain, art I hardware can make the room.

Sorry, I keep adding so many comments...

I know a lot of time "spa" can go towards "hotel" really quickly, and that isn't quite the feel I'm going for. I still want it to feel like home, but I don't want it to scream out "this is a new cookie cutter house in suburbia" the way it does now. I'm a big fan of simple sophistication and clean lines, without getting to the point where there is no personality left in a room.

I kind of like the darkest paint chip you have, like the crown molding idea, but if a new tap is not in the budget then crown molding is definitely out. That will cost more then the tap. Budget is always the biggest concern for most of us, adding artwork slowly is very affordable. Go browse in some liquidation stores for artwork and crafty stuff like vases or flowers, many times you will find towels bars and light fixtures, decorative towels, rugs.

I vote the darkest paint chip as well.

One way to spruce a bathroom is to use "real" area rugs instead of the fluffy ones with rubber mats. Another "trick" to glam things up is to layer real curtains (with your shower curtain) in a color/pattern you like, hanging from the ceiling.

I do like the darkest colour, but worry that it will be too similar to the floor and shower tiles and make the space feel a bit claustrophobic.

Before we painted, our apartment bedroom had yellow carpet, yellow walls, yellow trim, and yellow doors. It felt teensy tiny, but the room was actually 16x18 feet! Really worried about going down that path!

Oh, on the note of floor mats: bathroom floors are one of the more disgusting parts of a house, and I cannot handle the idea of putting something on the floor that can't be washed in super hot water. Those fluffy rubber-backed bath mats are the stuff my nightmares are made of! Hahaha

Try Sherman Williams Accessible Beige...

Here are some pictures using the "personal colour viewer" tool...

Either Frame the mirror or replace with a modern one. Some new knobs and new shower curtain also. The floor looks great!

I'd paint the vanity a lighter color maybe off-white with taupe undertones. Frame the mirror and hang some artwork above the toilet. Change the shower curtain with more sophisticated one...

Paint colors on Houzz are only suggestions. Paint colors looks so different on our monitors than live, in person. You'll have to decide (or get a friend) to help you on site.

They really do! I feel like they all look white in those pictures.

I'm going to pick up a test pot of the darker colour today and see if it comes across as too dark.

This is the darkest chip colour. Thoughts?

Chose the paint colour and am finished painting! Tape is still up and it's a mess, but you get the idea.

Going to install the new light fixture, a towel ring near the light switch, a toilet paper holder on the cabinet next to the toilet, and towel hooks on the big blank wall with framed pictures above them. Switching out vanity hardware to something with straight lines in a chrome finish. Still deciding between art or shelving above the toilet. Mirror frame is probably going to be longer term.

Suggestion,

1. tape off perimeter of mirror 2". Move in 3" more and tape again. Use krylon window frost spray on both areas. Frost as thick or thin as you like. Makes a nice but subtle border.

2. Unless countertop is stone, paint, yes paint, a neutral color. Highlights vanity, ties in new wall color, unifies flooring.

3. Use towel knows or hooks instead of bars

Hope this helps