You can actually make treat parcels out of toilet rolls
That can keep them occupied trying to get inside the parcel to get at the treats! Have a look at this video on 10 toilet roll toys by Erin's Hamsters. It's one of the cheapest safest toys you can give a hamster. They just love a tunnel to run through. Any cardboard tubes are fine. Kitchen roll inner tubes make a longer tunnel and you can also use old Pringles tubes (washed out).