I use traditional Roman dice games. Pick a number of dice (1-3), a number of "pips", and make a bet. If all of the dice are the number you picked, you win a certain multple of your money back. IIRC:
1 die - 4x
2 dice - 25x
3 dice - 150x
You can also make a combined bet, where you roll 3 dice and call a number; you win back your bet every time your number comes up. If your number was 3 and the dice come up 1, 5, 3, nothing changes; if 3, 3, 3, you get twice your bet back; if 4, 5, 2, you lose your bet.
*Wow* the odds are stacked, but the soldiers didn't have anything else to spend their money on...