Petrin Hill - Atop a hill on the Castle side of the river is a mini-Eiffel tower. We climbed the hill and then climbed the tower. Amazing views.
Old Town - The official "old town" and "new town" both fall in this category in my book. New town was declared "new" in the 14th century. It just is as great as you'd hope. Picturesque spires and red roofs everywhere you look.
Museum of Communism - A thorough look at the history of Communism in a country that was not "officially" part of the USSR. Really interesting.