I like mini-games. It's all a matter of it having balanced rewards--or, being so fun, yet rare that the reward is seeing the game itself.
What I've seen in other games, that would be nice(obviously, coding would be a different matter): Blackjack, poker, chess(yes, chess)[by extension checkers, or maybe even
Go(I don't even know how to play that...)]... there are other games I could list, but they're harder to explain... and would probably just be "stealing", and I don't think you want that.
Coin flips are kinda boring, though... I mean, it's nice to have for whatever reason, but it's not really so much a mini-game as it is just a way to make a 50/50 call.
Give us chess >P With an ASCII board and pieces >P (well, maybe just Alphanumerical designations for the pieces...) Chess is fun in and of itself without any reward. ... Don't know if anyone would -play- though. >P