Hamsters love fresh nibblets of sweet corn as a daily treat. I buy the small frozen corns and keep one in the small freezer inside my fridge. I break off about six nibblets from the frozen corn, place these in a cup of cold water and leave them for an hour. This defrosts them naturally. Then just refresh the water in the cup each day and use them up s necessary.
Apple is also a nice treat. You just want a thumbnail size once a day. Carrot is also fine. Dry porridge oats sprinkled into the food dish every other day will also be enjoyed. These are a good fibre source.
The rule of thumb is, if a hamster is just being introduced to fresh veggies, a very small amount should be fed just to make sure his gut doesn't react t having a change in his diet. Most hamsters won`t show any runny poo unless they are eating too much, so no bigger than a fingernail in size of each daily and go from there. x