I'd have to go with U1: Sinister Secret of Saltmarsh. its a lot of fun and has that "scooby-doo" mystery element and takes place in a haunted mansion. Plus, if they like it, you can continue it with the rest of the U series. My second choice would be B4: The Lost City, which has a classic D&D feel in a egyptian pyramid. You can't go wrong with either of them.
As mentioned on a recent thread Bone Hill (and a bunch of other adventures) are still available as free downloads from the WotC site. Bone Hill is a good adventure for young players, too.
2 Sessions.
Keep on the Borderlands
Village of Homelett
Against the Giants - G1
All of those could easily be expanded out to more sessions if its a possibiliy.
The Tomb of Horrors - if you want them to die.
White Plume Mountain
Expedition to Barrier Peaks
Both good for one shoting.