Shovel Knight: King of Cards is going to have a card game in it.

(Note that this campaign has yet to be released, but will be free to owners of Shovel Kinght: Treasure Trove.)

Sonic Generations included the original Sonic the Hedgehog (1991) as an unlockable.

Prince of Persia The Sands of Time also included the original Prince of Persia, but somehow not in the PC version. Oh, Ubisoft! :(

You're thinking KOTOR. The MMO does not have Pazaak.

Shenmue has SEGA arcade games such as Outrun and Afterburner IIRC

Whoops, yep, too many acronyms in my head!

I think there was a Spiderman game where there was an option to purchase games (with real life money) for Peter Parker's mobile phone.

Beavis and Butthead in Virtual Stupidity had the minigames hock a loogie, air guitar and some ant frying game

Beyond Good and Evil had racing minigames as well as an air hockey type minigame

Syndicate Wars had a minigame in a level where you had to defend some guy giving a speech, I forgot what it's called but you control a paddle at the bottom of the screen and you bounce a ball off it to destroy bricks at the top of the screen. I'm sure the game begins with an A

Metroid Prime (Gamecube) has the NES Metroid in it, but you need to use a special cable to link it with Metroid Fusion to unlock it. (I don't remember if you actually had to beat Metroid Fusion to unlock it; I do, however, know that you don't need to beat Metroid Prime for this.)

Metroid: Zero Mission (GBA) also contains the NES Metroid, and unlocking it simply requires beating the game (I don't remember whether Easy mode (which is too easy, by the way) is enough or if Normal is required; I know Hard (which is too hard IMO) is not required for this.)