Only give a piece of fruit as a treat as it contains a lot of sugar as said above, but also some are acid which is not good, never give tangerines, oranges, lemons, citrus etc.. even if some rabbits like them, too much acidity..
Sometimes, you can give a little piece of apple, pear, apricot, strawberry, cherry w/o the seed, banana but be careful it'"s very rich and contains starch which is no good for rabbit : I only use it as a trick, idem with currant to give medecine pills etc...