What do you want to do?
There‘s tons of great cultural places to visit -- real culture, that is, not just the "ballet" -- but when I was there it was more of a honeymoon thing, so we didn‘t get out of the hostel much.
One thing I can recommend is the Austrian Army Musuem. It‘s housed in a great pseudo-Turkish caserne-something built in the 1850s after they tore down the city walls. They‘ve got a fabulous collection of 16th and 17th century armour and weapons, as well as tons of paintings, statues and wall-size murals. It‘s up on the hill above the Schonnbrunn (sp?), a massive palace and gardens that makes Buckingham look like a granny flat. It also has a decent collection of vintage armoured vehicles, but nothing you probably haven‘t already seen -- or crewed.
They‘ve also got these great flak towers scattered throughout the city. They‘re almost 100 ft tall and made out of reinforced concrete 20 ft thick. Even though the Russians flattened most of the old city, these thing weren‘t scratched. It was too expensive to pull them down after the war, so they just left them alone. I think one is now a giant aquarium with killer whales and stuff, and another is chic-chic condos or something.
The innerstadt within the Ringstrasse is really nice with great old buildings, but the further out you go from the centre the grittier it gets.
And don‘t even think about trying to cheat on your subway fares! You buy a ticket from a machine and get it time-stamped when you enter a station -- no fare collectors. I thought this was a little odd until my streetcar slammed to a halt on an elevated line and two burly guys climbed on board and barked something I didn‘t understand. Everybody whipped out their tickets -- including yours truly -- but two guys jumped up and made a break out the back door. And ran right into Fritz hiding where he couldn‘t be seen. As the streetcar rolled away they had the two in cuffs going down to the street level!
I was a little antsy while all this was going on because I had forgotten my passport back at the hostel and had just dropped several thousand francs on gunsights and other target equipment. I figured that with my lack of German they were liable to call me a terrorist and throw me in the slammer. Luckily this was in 1999 and I had punched my ticket properly and so never had to open my mouth. Don‘t laugh -- it was a big deal at the time!
It‘s really a great city to hang out in though. The people are nice, the beer and food are fabulous, and I‘m sure the nightlife is pretty good too.