A good pinch of maui-wowie, rolled tight but not too tight...
Seriously though, put anything you like in 'em, I have used sliced mushrooms, roasted red peppers, cheeses of all varieties, garlic, onions, pre-cooked bacon. Porky made a good point that after you stuff them & you roll them back up & seal the seams you definitely want to put in the fridge for a couple of hours so the seams don't bust open when smoking.
Just a plain fatty in your favorite rub is always good too...