Millinaut is a great example of the kind of limited game that can be built in 72 hours. Created for a Ludum Dare game jam themed around a "tiny world," you'll play an astronaut carrying both a pint-sized spaceship and an entire miniature planet on your head. As you navigate a platform-based world, you'll need to swap the order of the two, take advantage of their special capabilities, and briefly separate from them in order to claim a series of trophies that represent... new galaxies, maybe? We're not really sure, but the game is both competently designed and very challenging (at least for me.)
According to Shaun Inman, one of the developers, the original plan was to have the player control an id, ego, and superego while "wandering around inside his own head." The concept was changed, but the threefold totem remained the same, this time in a more classic setting than the envisioned "dark, psychological drama." Even if you don't play the game, it's worth reading both the postmortems linked above to see how it was developed by a team of three people, in two different places, over three days.