it could be something like steam cards for sure. For each achievement unlocked. As example, players could receive a free NFT minted on the Unique Network Testnet with free tokens (named Opal).
If dealing with this is too complex, you can add the achievements or other NFT-like elements in the game (collectables, characters, etc), but not the actual blockchain stuff and we'll help with the integration, even after the game jam ends (if the game is good enough) to make the game qualified for the prize.