Buttons are the most popular, although teams will do different things. We do bouncy balls AKA: Mini Mayhem Makers (they really do and we have gotten yelled at!)
Some teams will find other cool handouts: keychains, pens, mini tools, rubber bracelets, light up pins, bead necklaces, you name it and it will be at a competition. Don't know where to get some of these items.
Good luck!