I just moved into a rental too, so I'm looking for the same things. There are removable stick-ons, even wallpaper, that can help a lot. I like the "wall words" stick on words and phrases.

My friend took her moving boxes, broke them down and made a huge, flat piece that covered one entire wall of her bedroom, covered that with wallpaper, and tacked it up to her wall with those Command strips. You can buy the strips separately in packs, or just use tiny drywall tacks that don't leave big holes. She's not really allowed to hang a lot of pictures that leave holes in the walls, so she got a piece of lattice fencing, painted it a really pretty complementary green color to go with her LR furniture, hung it from the ceiling on plant hooks (which she is allowed to have - go figure) and hung her pictures on that. Makes a unique and pretty wall.

In my kitchen, I have a washer/dryer closet that has no door. I'm going to make a "door" out of 2x4s and pegboard, with one side to hold my pots and pans, and the other to hold spices and utensils. I'm not going to "hang" the door, just put it on rollers, so I can roll it out to turn it around easily when I need to, and to allow the W/D to be moved in and out easily if necessary. That will free up a lot of cabinet space in my tiny little kitchen (the only part I hate about my apartment). Also, there is a huge opening into the kitchen (sort of open floor plan), so I'm going to put small, narrow shelves up each side and across the top. The LR side will be shelves to hold knick-knacks and the kitchen side will be pegboard to hold other things.

My kitchen cabinet doors under the sink have water damage, so I'm removing them and putting a nice "skirt" there under the sink. That way, I can easily take it down to wash it if it gets dirty. Since my kitchen has no window (bummer), this is the only way I can add a kitchen curtain of sorts.

you could use a temporary wall covering product called stickwood - we have seen it in a few houses and the produced has a good review. it also has a fire rating for safety.