Budapest: A tour of the Palgovy Caves, shopping at Nagycsarnok, shopping at the Chinese Market (with super cheap stuff to buy), seeing the House of Terror (that has a pretty crazy history), or going to the Basillica of St. Stephen.
Vienna: Definitely going to the "Cafe Central" (which has some awesome history to it), and maybe the oldest zoo in the world, Leopold Museum, other sights, or Schloss Schonbrunn Gardens.
Prague: Definitely walking across Charles Bridge and shopping on Golden Lane, and possibly the Mucha Museum, sights around the city (including the Astronomical clock!), or this antique musical instruments shop.
That's all I've got so far for ideas, I'll be in Prague for three full days, so I'm trying to focus my energies towards finding fun stuff to do there. Anyways, I'll have to let you know how it all goes later! Still debating on whether or not I should take my laptop on the trip, but worste case, I'll at least post some of the pictures afterwards of the trip :) (I STILL need to post pics from the four day trip! :P )