I was going to say "just sneak in a joint man" but since you mentioned brownies, I would DEFINITELY do that. Just eat the brownies like an hour before the band comes on, or even more. Brownies are supposed to last like 6 hours or something, don't know since I have yet to try them. But I definitely suggest eating some brownies.

And if all else fails, I'm sure you could smoke a J in the parking lot.