One museum we've always wanted to visit in Pittsburgh, but had few to no opportunities to check out, is the Children's Museum. As adults without kids (or kids we can adopt as defacto parents for an afternoon), we always felt uneasy with the idea of going to this one on our own.
Our problems were solved when one of the museum's quarterly 21+ night events came about, and we put on our detective hats and went to work as the Murder Mystery theme ensured that all of us became a kid again- for one night at least.