There was a poker game - Governor of Poker - where you were making money to buy out whole Texas (I may exaggerate here), that was a funny idea, because it was giving you an objective.
Another thing that poker reminds me is strip poker! That's a great way to motivate players - not only in poker, stripping girls are welcomed everywhere (well, maybe not on a minigame made for a site about feminism). Just remember to comment this answer with link to your game when it's finished, so I don't miss it.
Some people simply implement score. It's especially nice if you can share it with others, and you will rather want to share "Yeaster Master" in a beer promotion game than "1539205", so coming up with funny rank names may pay off.
Similarly to above, a lot of games nowadays spend time on creating achievements, especially if you can share them on your blog, Facebook, forum signature etc.
Also a great reward for a player is new content unblocked - usually it's just a step forward in story of a RPG game, though it works also with lesser rewards in low budget games, like unblocking Hold'em variant in a poker game.
If you have other products, you can put some secret code for another game. Hackers won't take it from decompiled code, because they won't know it exists.
Edit: Krom Stern pointed out in comment section, it might be helpful to put more examples about the >>content unblocking<<. He's right, that the term "content" is vague, as it should be. Basically anything is content: soundtrack, background, model textures and game mechanics. If you, a player, a modder or a random generator create something - it's content. It may be counter-intuitive what kind of content may be a good reward. Some developers think only a content involved in game mechanics will meet player's satisfaction: a trinket increasing luck, an hourglass able to rewind some unfortunate events (drawing bad cards) etc. However, there are a number of "unlockables" invented by designers throughout the gaming history. Here are some relevant to a card game example supplied by question's author:
- Codex - a book with trivia, where some facts (or parts of fictional story) are revealed,
- Avatars - very rewarding if a forum or chat is connected with the game,
- Card backs, as practiced by Blizzard in Hearthstone,
- Similarly as above, alternative front look of cards could be a reward,
- Different table, token, player avatars (as in heads near the table, representing the players) textures
- Steam, Kongregate and other badges etc.
If you run out of things (or rather categories of things) to unlock, then you can also add another layer to the game, as in the game mentioned in first paragraph. For example, you could have a living room with space for various vanity items that represent your wealth in a more rewarding manner than a number.
