rabbits are vegetarians and their digestive system is very different from ours. they are not set up to digest carbs, even the sugar in fruits.
of course they love the fruits, they are just like us when it comes to sugar! but we have to be very careful with treats so that their digestive system doesn't get overwhelmed with the carbs and starts shutting down.
some bunny people give treats of grapes, raisins, banana, strawberries, blueberries etc. the trick is to give tiny pieces (bunnies don't know sizes, if you give them a whole slice of banana they thinks it's the same a 1/4 slice) and to handfeed. treats are great for strengthening the bond between human & critter.
still, treats should never be given to a bun who shows any signs at all of eating problems. s/he needs to be eating tons of hay, all day & all night, and also a good helping of greens. pellets need to be rationed (timothy pellets 1/4 - 1/2 cup/6 lbs bunny weight for adults; babies up to 6 months can have as much as they want of alfalfa pellets & hay). also, make sure s/he is peeing & pooping normally (there we go again with the poop thing!)
in general, greens include pretty much anything except for iceberg lettuce (bad for humans too!) and cabbages (green, purple, cauliflower, broccoli). good examples would be chard, bok choy, parsley, cilantro, green pepper, red/green/butter/romaine lettuce, dill, mint, basil, dandelion, kale, carrot tops etc)
what i do is different from most bun moms and i have taken flack for it! friendly flack. but i am a preemie r.n. and i totally believe in prevention and good nutrition; for me, by the time one of my bunz is showing symptoms of a g.i. issue, i could have caused it by the sugary treat i gave yesterday...and that would freak me out too much.
so, for treats my furry babies get alfalfa cubes (taste better than timothy but can only have small amts), oat hay, oxbow papaya tablets.
i also give certain of the herbs as treats: basil, for instance, as i only get it fresh and it wilts in 2 seconds. mint. and when i give them their regular greens at night i handfeed them the first few bites cuz it's lotsa fun!