That is pumpkin seed
Completely safe and they provide good protein and healthy fats for hamsters!
It really depends on the individual hamster in terms of extra foods,but you do need to be absolutely certain that the food in question is completely safe and healthy.This means the majority of storebought treats will be a big no,but there are still many potential safe treats for hamsters.
Personally I just end up using mostly people food(freeze dried fruit for example can be a good treat,but feed in moderation as dry fruit can be very high in sugar compared to fresh fruit!).Flax seed as an extra is wonderful as well,or for example you could make some sort of DIY treat cooking rice,barley,and other things together and then freezing small portions and feeding a tiny hamster-sized portion at a time once a week.It's fun,usually cheap,and very simple as long as you can get creative with your recipe!