I mostly make trivial changes, such as spelling or punctuation. On occasion, I will add additional details to existing articles that cover subjects with which I am personally familiar. I enjoy reorganizing articles that have slowly built up a lot of good information that isn't easy to follow.
Aside from Wikipedia, I am still sporadically active on wikiHow, a wiki How-to manual, where I was made an admin, though I have been putting in more time lately on CreationWiki, a wiki on science from a Biblical Young Earth Creationist perspective (where I was also given admin privileges).