I have given my Syrian Gloria a small amount of watermelon before, she loved it and she made herself a sticky mess which kept her busy for a good 20 mins cleaning all the juice out of her fur
I have read that hamsters are discouraged from having milk in their diet because, like humans, they have the potential to be lactose intolerant. Lactose intolerance is more common in cultures where milk is not consumed beyond infancy (example- some asian cultures, according to my nursing school text books). I don't know of anyway to predict lactose intolerance in hamsters.
I used to give Gloria 2-3 mL of skim (fat-free) milk with her vitamins mixed into it once every few weeks as a treat. I gave it to her through an eye dropper and she loved it