If you can't use nuts, what's a good alternative?

I think sunflower seeds are a very good idea or you can also use pumpkin seeds.

i like to substitute sunflower seeds in things like pesto and granola, since i have a tree nut allergy, but in this i'm not sure if the flavor would be exactly right... probably depends on how much you like or dislike the flavor of sunflower seeds! good luck!