What is your favorite brownie mix-in?

I'm going to bake some brownies from scratch but was thinking I may spice them up a bit by throwing something in. I know traditionally walnuts are thrown in but I'm having my sister for dinner and she is not particularly fond of them I have thought about: mini marshmallows and graham pieces, m&m's, chopped reese's cups, or candy chips of some sort. What works very well and what do you find most appealing in your brownie? Thanks for helping me out!