I've lived in Vienna for three months for school. Be prepared for a great experience there in Austria. I'm assuming you've brushed up on your German, and be prepared to bring a fan, since most places don't have A/C. Now for things to do. . .
If you want the small town experience, go to Salzburg. That's where Sound of Music was filmed. The Mountains are GORGEOUS!!!
But Vienna is what I'm most familiar with so I'll focus on that. I would recommend visiting the Schonbrunn Castle, a fantastic culture experience. The Sigmund Freud Museum is there; a very small location, but worth finding. And you're going to have to go see at least one opera or ballet at the Oper in the center of town. You can get Standing Row tickets for 15 euros and get a great view. And getting around the large city is very easy. The subway offers week long tickets for 25 euros, and is very very easy to navigate. In fact, you will find Austrian cities have an easy to understand Public Transportation; so don't worry too much about renting cars or taxis.
And also for food, I recommend going to the Keystrasse. That's where the Sacher Hotel is; where they make the Sacher Torte; a chocolate cake unique to Vienna. Plus, that street is where alot of good restaurants are, as well as Food Stands, if HUGE gyros and Danishes are more your thing. Also near Keystrasse is where you'll find the Stephansdome church; and centuries old catholic church with a BEAUTIFUL, mysterious interior.
I'm sure I'll think of more notes, but most would be described in your guide books already, and if I was to think of anything more, it'll likely take up the whole forum. Just go there and have lots of fun.
Oh, and enjoy the parks too. Especially in the Schonbrunn. The flowers and trees are very nice this time of year.
And I should also mention that Austria is where Red Bull is made. You'll find them in every vending machine.