We currently have a simple slot machine in our online virtual world game.
You set the amount you want to bet (in virtual money) and pull the lever, and depending on the 3 figures (there's only one line) you get a reward, a free round, lose some more money, or most of the time, get nothing at all. You can instantly pull the lever again. The previous bet will be maintained.
At the moment this slot machine isn't really fun to play. The chances of winning something are pretty low (though when you win, you can win a lot). When you pull the lever, the result is determined (though not shown) immediately, so there's also no way influence the outcome of the wheels.
How can it be made more fun?