How can I change the look of a room without painting it?

You can add wall decals. You can just get designs, like trees or flowers, or get a whole wall mural that is peel and stick, or just one large mural, like a big clock or something. You can use washi tape to create stripes on one or more walls. You can also add pops of color with new pillow covers, throw blankets, flowers in vases. You could hang a large tapestry as well. Here is a fun idea as well:

https://www.forrent.com/blog/decor-for-the-home/renter-friendly-3d-paper-wall-art/

Use color in your accents....curtains, throw pillows, rugs, fabric on furniture, lamps/lamp shades, knick knacks and books in a book case. Just think of your white walls as a canvas! Have fun!!

It is all in the decor and accessories. Do fun curtains , use furniture to add height and color. Look how things are styled on Pinterest. Pull all your stuff out and see what you can do with what you already own. Grab a friend or more and see what you come up with, even rearranging can make a difference. Throws, pillows, candles, plants all help!

What about repositionable wall paper? or vinyl decals?

Hello Rosetap62 :) Contact paper ... I'm serious. Today's contat paper isn't heavily sticky like it used to be years ago. It sticks where put but is easy to pull off and usually leaves no residue behind when you heat it with a hair dryer as you remove it (or very little IF any and easily removed using Goo Gone and a plastic scraper). Folks are using contact paper now for walls, counter tops, backsplashes, cabinets (inside AND outside) and even for flooring! The first lik tells of 5 different ways you can change up your walls without using paint:

https://www.forrent.com/blog/decor-for-the-home/5-ways-to-color-your-walls-without-paint/

https://www.apartmentguide.com/blog/no-paint-allowed-5-options-for-temporary-wall-coverings/

https://www.viewalongtheway.com/2012/08/introducing-reader-questions-5-renter-friendly-temporary-wall-covering-solutions/

and last but not least ... I really like the ideas here... and pics ;)

http://www.apartmenttherapy.com/10-temporary-removable-products-all-renters-should-know-about-206028

OR you cold do like SOME renters have done: say to yourself "Well, I DID pay a security deposit so letthe landlord change things back with that after I move out." That's not very above-board and COULD get you evicted, but they ARE going to keep your deposit for cleaning and repairing anyway once you move out, no matter how clean you keep the apartment.

You could put up fabric (just like wallpaper) using starch water. It was a trend from years back. All you do is soak the fabric in starch water, squeeze out excess, and hang. Pulls right off when you want to take it down, with no damage to the walls. And, here's the good part, you can wash and re-use the fabric.

Fabric soaked in Liquid Starch and applied to walls make a wonderful temporary wallpaper. It comes off easy and does no damage to walls. You can use flat sheets or other coverings.

wall tapestry they supers cheep