I mix the Science Selective biscuits in with Burgess SupaHamster for one of my mixes.
My Syrian is quite fussy and he won't eat the bickies in the Burgess, or the pellets, but he really likes the Science Selective so I give him a few of those, as then I know he is getting his required nutrients even though he's quite selective.
The SS is a compete food, so they are getting everything they need in one biscuit.
For other treat ideas, here's a few that my lot love:
Millet sprays (you can find these in the bird section of most pet shops)
Peanuts in shells/monkey nuts
Dried goji berries
Dried banana
Pumpkin seeds
Dried forage (dandelion and herbs)
Dry porridge oats
And BUGS. I have yet to find a hammie that does not love a fat juicy bug! You can give mealworms, crickets, locusts, and of course... the ultimate favourite...
waxworms. My lot couldn't get by without a plump, wriggly waxworm in their diet!
You can find these in the reptile section of most pet shops. Most are sold live in little pots, but some you can get dried or canned. You can pick up dried mealworms quite easily, as they are sold in loads of shops for wild birds.