Here is a great way to add some pizzaz to a boring wall in your office or guest bedroom from Readymade. They give you directions on doing the whole wall in posters which is super easy and fun!
But if you don't want to do the whole wall, a really interesting thing to do in a guest bedroom could be to paste the posters and artwork up behind a bed in a rectangle and make it into a fun "headboard." It's a grown up version of the wall you had plastered with posters in high school. The possibilities are endless, get creative!