Almost a year and a half after we moved into our apartment, we still have almost nothing hung on the (bright! white!) walls. In a fit of industry one day, I hung the four framed pictures we own, plus a small row of hooks for CS to hang his jackets. That was two months ago, and the rock hammer that I co-opted for the task is again collecting dust as I contemplate the situation. I have decided to tackle CS's room and play area first. He is the one most likely to be truly delighted by having something colorful on the walls, and also the least discerning about what that something is. I have been putting some thought into both of these areas already, as CS is now to the age where he will spend time playing by himself, and he seems to like to do it in his own space.
For one of the walls in the bedroom, I think I will get some stretched canvases from the local Pier-1 type store called Casa. I think that they go for about 2 euros apiece. I am going to experiment with these and maybe turn it into a toddler craft project. I will paint the canvases with wide stripes or solid blocks, then do some sort of collage on top. As I type it, it sounds kind of ugly, but in my head I think it might work out. I have a lot of the dustjackets that I have removed from C.Sam's books, so maybe I can cut out the favorite characters and incorporate them in some way. If I had access to a good thrift store, I would go looking for old wallpaper or something to use as the background, but alas I do not know where the thrifty Spanish go for their finds.
The play area is a bit of a challenge because on one side is a curved wall/alcove made of frosted glass from floor to ceiling. Great for letting in light, but hard to hang anything on. The other side is a wall with four doors and a shallow nook. I have been looking at wall decals for this area. Below are some of my favorites.

These are from Blik Surface Graphics. On the left are some the of the designs from the Wee Gallery, which has bold black and white animals with colored accents. On the right is an alphabet design from the Threadless bunch. We have a heightened interest in letters and numbers lately, and paired with animals, how can you go wrong for a two year-old?

These skyline decals are from Zuuz. I like the chalkboard idea, but I would love to have one in a color other than black. These buildings are modeled after Amsterdam. It would be fun to design one of these for our own skyline - these guys do custom designs, though they are a little out of the current budget.
I think these alphabet decals from Wonderwall are my favorites right now:

I think the "elefont" is adorable, and the "a,b,tree" would look great on one of those frosted windows. I wish there were more font animals, but it's probably for the best, as I can't sink the whole college fund into wall decals. Right now I'm thinking I'll get the tree for the play alcove, and the elefont for the other bedroom wall, paired with some numbers from Zuuz to make it look like he's spraying them out of his trunk.

Zuuz and Wonderwall links via Bloesem Kids.