You can now buy some squirty stuff to put in kongs, but you can also use peanut butter and stuff it with bits of biscuit or dry food.
If you buy one of those balls that have treats stuffed in them, make sure the hole where you put the stuff is small otherwise your dog can get its jaw caught in the gap.
I never found the spray works and unless you spray every surface, wire, article of clothing, shoe, etc. they will find something. Better to confine them and give them stuff they can chew without causing a problem.