Paint, furnishings all help and don't forget about lighting.
Which room are you wanting to redo? Post a photo if you can, so that we can help
Here is a site with some stunning ideas. The monochromatic look with the decals on the wall is beautiful.
Here are 3 different painting techniques that add depth also:
How to add depth with accent color: https://freshome.com/2011/06/14/how-to-add-depth-to-your-interiors-with-accent-color/
Create the illusion of space with ceiling color: https://www.homedit.com/creating-the-illusion-of-space-with-ceiling-color/
This site focuses on color, lighting & mirrors: https://freshome.com/2007/05/29/5-tips-for-fooling-the-eye-and-making-a-room-look-bigger/
Mirrors or dark paint on walls that have windows so you eye goes outside. I did dark green & it’s stunning but you need a nice view.
I like to layer my decor to add depth! Here's some tips: https://www.aratariathome.com/blog/tips-for-layering-decor