You should have some Double or Nothing. All you need is /coin. If they get tails, you keep their money, if they get heads, you double the value of their wager. They can walk away at any time or try to double it.
Except Jack, that is a Zero-sum game. If you're familiar with actual casinos there is always a "house edge." A double or nothing game would never make any money for the Casino. If the payout was like 1.5x your money (so 1 million reaps 1.5 million) for a winner it would work. Or an alternative would be to charge a base fee (say 10% of a wager) on top of the actual bet to produce the same edge effect. Not looking for it to be a huge money maker, just don't want to go broke running it.