In most of our apartment, we tend to put up a lot of photos taken from our travels. I find those always bring good memories back, and remind me that experiences and memories, not stuff, are the best things to spend money on.
San Sebastian, Spain by
Thomasaurus, on Flickr
Amalfi Coast by
Thomasaurus, on Flickr
Museo Casa Enzo Ferrari by
Thomasaurus, on Flickr
In terms of where I spend most of my time, my office, I use my walls to test out my own work to see how it works at scale. My walls can get littered with printouts, which in itself is kind of art like I guess.