my friend Megan's 2 year old son James (who is adorable and very active) left her a gift on the sofa yesterday.
Luckily the sofa is scotch guarded and the markers he used were washable, but just in case I sent her this article from the nytimes and told her that it is one of my favorite house ever because it is.
Pamela Bell had her daughter's 7th grade class cover her John Derian sofa in fabric marker graffiti, and let her son have his way with fabric paint on a couple of chairs. I love this idea so much and when we have kids I plan on giving them some chairs to draw on. My only concern would be that they might get confused about where and on what it is appropriate to draw at other people's houses.