I would like to know some different ways to treat my flat bath wall.

You could tile it. It's not that hard to do but does require some time. You could go with all one color or you could make a design. If you don't want to do the tile job yourself, you could hire a pro from Home Advisers.com. We recently had tile installed in our kitchen. We purchased the tile, glue and grout and hired a pro from them to install it. They do background checks on their pros and provide customer reviews too. And, here's the best part, it's free. We saved money by purchasing the tile, glue and grout, and we were very happy with the pro who installed it. Win - win.

If you want something that looks like wallpaper, you could do a pearl strie wall using pearl metallic paint and a wallpaper brush... https://static1.squarespace.com/static/571f755a01dbae7f62f292f8/571f80b11bbee028fadfeae3/5720ede32b8dde45863330d1/1465346456360/ariana-hoffman-westfield-nj-strie-custom-wall-treatment-painted-dining-room-wallpaper-jeffries-alternative-washable-durable-repro-designer.jpg?format=1000w

https://www.do-it-yourself-help.com/faux_painting_strie.html

I have also done this techique using tulle netting on almost dry paint to get a heavier strie..... https://thishandcraftedlife.files.wordpress.com/2012/05/strieyellow.jpg?w=529&h=805

You can also do stripes in a monochromatic color scheme using either a flat or eggshell paint, and the next stripe in a satin or semigloss finish in the same color.... https://guideimg.alibaba.com/images/shop/79/09/24/6/free-shipping-simple-wall-paper-stripe-wallpaper-study-living-room-bedroom-plain-color-wallpaper-papel-de-parede-0-53m-10m_1981866.jpg

Shiplap is easy to install with a few tools.

Hi Laura, Here's a link that may help: https://www.thespruce.com/best-bathroom-wall-surfaces-1821359

Good luck!

https://www.hometalk.com/categories/makeovers/bathroom/shiplap-walls

I would try to address the humidity before doing anything else. Is there a way of changing out the ceiling light to one with an exhaust fan? Maybe that exhausts into the attic if that is OK? If that can be done, you'll have no problem installing wallpaper or anything you'd like. Best to you.

If needed, paint in a color you like. Find a nice large print / picture at a thrift store and cover the wall. Large wall art is available for under $20.00.