My buns basic diet is hay and pellets, so anything that differs from that in terms of fresh is considered a treat (so for us it would be basil, dandelion leaves, parsley, mint, and then the fruits, pear, apple, banana), also I buy special hays as a treat.
Remember whenever you introduce a new food to do so slowly, and start with a small amoutn and increase it over time watching carefully for any signs of an upset stomach or any other problem.