Lake Bled (pronounced Blade) and Vintgar Gorge are well worth a day, you can do it as a day trip fro Ljubljana very easily.
Budapest is one of my favorite cities. Get a place in the city center so that you can walk everywhere. There is a cool museum whose name escapes me that covers life during the nazi and communist regimes. The castle is of course cool as well as the surrounding historic neighborhoods. I actually prefer Budapest to Prague for the architecture, affordability, and more genuine feel.
If you haven't made travel plans yet, think about taking the hydrofoil from Budapest to Slovenia. It takes about 2 hours, and you pass some very nice scenery along the way up the Danube. You can also catch it to Vienna, which is a great city to see as well.
In Budapest, I would visit the Roman Baths: ROMAN BATHS Budapest has been celebrated for its curative baths since Roman times, when the city was known as Aquincum. Among the most venerated are the Szechenyi baths (36-1-363-3210,The Link which sit atop a metro station (Szechenyi furdo) in the middle of City Park. Occupying a sprawling, neo-Baroque complex from 1913, Szechenyi has 15 pools ranging from frigid to steaming. Entry fees start at 2,400 forints. Be sure to bring a swimsuit: unlike many segregated baths in town, Szechenyi is co-ed and has a G-rated family atmosphere. Also check out the following link: The Link
Ljubljana is a cool little town. The Ljubljana Castle is worth the trip. Great views of the city and interesting history. I'd also recommend the Vinoteka wine cellar (also in Ljubljana) if you're into wine - great selection and you can do a multiple course dinner with lots of samplings. You'll notice horse on the menus around town...I actually got halfway through a "colt" steak before I knew what it was - thought the waiter said "cold" steak. Wasn't bad, though - at least you could say you tried it...
