There are a whole range of treats listed on Buff Hooligan's (I think it is the correct person) page.
One of my girls favorite treats is the organic greens we planted in the fall and let them stay all winter--turnips, collards, kale, rape, mustard, radishes, etc. I would pull them up by the bucket full and stuff them into their suet cages every morning... gone by lunch...
I also give them premium birdseed that has nuts, berries, oats and seeds (no corn), that has a protein of 11%, for them to scratch around for.. I purchase live crickets at the bait shop and mealworms they LOVE them...
Last summer, they got all our tomatoes, squash, melons, fruit and berries that were not fit for us.. bugs, bad spots etc. and for the first time I didn't mind 'discarding' those fruits and veggies!! When we harvested our corn--they got all the worms/buggy ends.. we don't use pesticides or chemicals...so it was a win-win ...