For festivals, it's fun to add effects with a Subpac or bass transucers.
Pros: - Bass transducers convince people that the ground is shaking and they usually don't expect it, so it's an exciting surprise. - Subpac adds a lot of rumble to your core, which (again) people aren't used to and it's exciting for them.
Cons - Subpac is hard to take on and off if you aren't used to it. Battery runs out quickly.