Bunch of good idea's already mentioned! We use the Tuna in our feeder buckets, they come looking for it real good!
Have also used the Marshmallows, they love those too!
Pretty much anything you use will get them. I've done almost anything imaginable.
Old Stale Cookies
Old Cupcakes and Cake
Baked Potatoes - They Love them!
Hard Boiled Eggs
They are not picky!! Here's what I do that seems to help the ones that have already been trapped.
Leave a trail of goodies to the trap. Just a couple little pieces and about 5-10 feet from the trap and then another small chunk right in front of the trap. THen place the rest in the back behind the flap/trigger. THey'll get a taste of what you have then get greedy and go after the rest in the trap.