Anything you like! I have a whole cupboard dedicated to my Syns, ranging from a Reeses Peanut Butter Snowman (a whopping 36 Syns for the whole thing) to Choc Shot (just half a Syn per tsp).
It really depends what your willpower and self control is like. Can you keep "normal" things in there and eat them in moderation? Or do you need to restrict yourself to lower Synned treats, i.e. the Freddos, Curly Wurlys, etc.
Essentially you can put whatever you want into your treat tin / stash.