For the rewards still people will prefer with the bitcoin, for additional rewards seems like ideal is the free spins, or bonuses for the events.
You can run too an event like bug bounty hunting to help your developers check what the possible hole in your system. If you want to have unique reward have a physical reward like Hardware wallets to the top reviewer
Indeed, most people here for prizes of reviews are Bitcoin which is the best payment tool, I see the average person here choosing Bitcoin as a gift payment.
Free rounds are quite interesting for a gift, but I don't think it will be effective and if you think of who wants a free round that has no value or with a small bet.
I think it must be done officially, and or hire a manager to manage it so that everything is well controlled, but the impression is like a contest, I don't expect it like that.