As for XP, I am thinking of awarding it for several categories, since it won’t be a fighting based game.
1.How well they make their performance checks for each song.
2.Proper song arrangement.
3.Cool new song lyrics.
4.If the players themselves sing a few verses of the songs (likely requires a lot of beer).
5.Fighting, of course.
6.Successful theft .
7.Getting laid (some sort of dice roll required to score), extra XP if it’s a nobleman’s daughter or some such target.
8.Roleplaying the characters well.
9.Living the lifestyle of a modern rock and roll band.
10.Gold, magic or other treasure gained.
11.Battle of the bands victories