Thanks, but I'm just in idea mode right now. I'll reach out to a lawyer when I have done more research.
I found something like this: https://pointvoucher.com/About "Consumers can download exciting branded mobile games and play their way to real products and experiences for the time they spend interacting with the games. Vouchers can be claimed in the branded voucher shops connected to the game or in the Pointvoucher shop where they will be exposed to games from other partners."
The experience is a bit complicated in the app though. You have to separately create an account on pointvoucher, which I'm not a fan of.
Any other ideas people?