Well, you can always sing in Quickplay and throw up a set of songs for fun. The scores will still count towards the leaderboards if you have a great run in a song. I've played a ton of the challenges, too, but I have a lot of really long setlists left there (including some of the "infinite" ones. At least you can save part of the way through the set.
On the BWT, I've been doing a mix of things. Playing drums some, playing guitar/bas and singing...whatever I'm best at for the venue/set that I'm working on at the time. I sold out to Hot Topic and lost fans. Of course, I went back to that song and tried to do better later without recalling that it was the Hot Topic set. Didn't mention it until the song was over, then I suddenly lost 700,000+ fans. Bummer!