Both Prague and Budapest are VERY nice, easily to navigate cities. And a lot cheaper than Vienna.
If you have only one day, you should choose between the two based on your interests:
Prague: Nice castle, beautiful old city square, some great photos, very nice Opera house (easy to get tickets), the Charles Bridge, earlier Effiel Tower, many Art Deco buildings, city of a thousand spirals
Budapest: Beautiful Opera house (has tour as tickets are hard to get), multi-mineral (thermal)spas, cafes (coffee houses), Terror Museum, Statue Park, Andressy Ut & Franz Litz Ter. Has onion domes.
See Rick Steves DVD on Budapest maded about 5 years ago. That is how the city looked then.