Try the gameplay of mario, and make it a web-based. The challenge is to reach the end of the mission, this mission has so many complicated obstacle, to enter the mission, the user must pay first to play, if the user reach the finish line, his payment will be doubled or it will depend on you on how much prize you will give.
Mario is not a good suggestion. I suggest OP to better do something like slither.io and the one that will reach a certain score will take the price, you can also add a consulation for 2nd and 3rd runner-up.