Yes, the card game. No, hear me out!
As with many classic programming challenges, there's a deck of cards (usually several!) and shuffling and perhaps sorting involved. That's always fun. And there's all the cardshark jargon :)
At its most basic, there's the trickery of an Ace being worth either 1 or 11, so you sometimes need to track two possible scores for a hand.
Then there's the asymmetry in that there are two "players", but one is the dealer who must draw cards until a score of 17 or higher is reached. Perfectly simple rules for a computer to follow.
Now, while Blackjack is basically about getting closest to 21 without going over, there are many other rules that can be respected or ignored as one sees fit (the title above links to Wikipedia's article on Blackjack, where you'll find lots of stuff).
For the more adventurous there's "splitting": A hand of two cards of equal value may be split into two separate hands. And, depending on the hands and the rules you use, you might then split those two hands again. And while face cards are all worth 10, some rulesets do treat them differently when splitting.
Also, the original "blackjack" meant specifically Ace of Spades + a Jack, but nowadays it usually just means Ace + 10. And by the way, a blackjack is worth 21, but still beats non-blackjack hands worth 21, so straight-up numerical comparison isn't quite enough.
And then there's doubling down: Doubling your bet and receiving exactly 1 more card, but no more.
There's also "insurance" which is a side-bet on the dealer having a blackjack. This makes more sense in a real-life game where the dealer draws a hole card, and the player can attempt to "read" the dealer like in poker. Here, the dealer will probably be your inscrutable computer (since it's no fun to play as the dealer), so insurance probably isn't that interesting. Unless your program is counting cards, perhaps?
In terms of overall scope, there are many options:
A full game with a dealer and 1 or more positions for the player to play and configurable rules.
A game that plays itself, learning the best actions to take for a given hand over time, maybe? Tables for the best strategies for given hands already exist, so you can check your results.
A card counting program that plays the game like Rain Man. It can just track the most likely card to come up next, or think several moves ahead, including splits and doubling down, or tricking the dealer into busting. Just don't bring it near a casino if you value your health.
A simple game of blackjack against the computer (just hit/stand; no splitting/doubling down etc.).
A simple scoring function for two or more cards.
And anything in between.
In all, there's should be meat here for any skill level. The house wins.
