Yeah, food is kind of a gray area, it would either really be appreciated, or really looked at askance. I think it would work better if you were either known at the venue so somebody "official" could vouch for you, or if it was a really intimate show with just a few people where you could share with the audience a little as well as the band.
I think the idea of a gift, though, is just fine, but darned if I can think of anything that would work without knowing particulars about the band and what they might like.