I get my bunnies the freeze-dried fruits, with no added sugars. They love them. Rabbits cannot have too much juice, or fluids as it will give them the runs. So the freeze-dried gives them all the goodies, but not all the sugar juice.
Some of the ones they love are pineapple, strawberries, banana, and papaya. I buy the brand Mother Earth on Amazon.
I also get them apple sticks, Timothy cubes, and other chew treats. The trick is to get them to work for their treat.