Buy some new artwork. An inexpensive colorful print with a sea turtle in it would be fine. Change the mirror if you really don't like it.

Keep the basics of the bathroom more adult like, but add peel and stick decals inside the tub area where they'll be out of sight when the curtain is closed.I would remove SPLASh, and add a nice rug for some color, picking a shade from it for the curtain.

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You should be able to find something cheerful and full of color that still feels grown up enough for guests. The flooring is very neutral.

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You didn’t mention redoing anything so I assume that is not an option at this time. My eye landed on the yellow step stool so I would add accents in that color. As in towels and art. Maybe you can replace the mirror over the vanity. You could have towels just for the family and separate towels just for guest.

I would go with high hung white curtain. As higher as you can.

I always use white towels. Bleach-able and always fresh looking.

What don't you like about your vanity and mirror? How do YOU define bright and happy? If this were my space, I'd remove 'Splash' and get a bright fabric I loved and hang it at ceiling height as a shower curtain. Hung from ceiling to floor outside the tub, it would make a huge focal point that would distract from anything else I disliked in the room. (Keep the other rod for the actual shower curtain.) I agree with cpartist about the backsplash. It doesn't jive with the rest. I'd probably change the mirror to a round one similar to this and get my kids' artwork on the wall. Seriously one of the cheapest and most personal ways of adding colour and art to a home!