Since folks know that I regularly visit Vienna, I get asked from time to time for ideas when others are making a visit to the city. First of all, take a look at the Vienna basics page. Then make sure to consider the ideas on the short visit page. But if you only have some extra time, you may want to consider these as well...
Swim at Bad Oberlaa
This one is a bit out there, on the far south side of Vienna. Give yourself at least a whole morning or whole afternoon. This complex is a spa, but the pools are pretty much stand-alone and a lot of fun to visit. Very warm water. Very peaceful experience. There is a large tube waterslide for the kids too. Kurbadstraße 14, (take the 38A bus to U4, U4 into town to the U1, U1 to its endstation at Reumenplatz, and then the 67 from there out to its endstation at Oberlaa). Expect to spend some money, $40 euro or so for four if you rent a private "cabina" to change in.
Go to Durnstein
OK, not Vienna, exactly! But you can make an easy daytrip out of Durnstein. Early in the morning take a train from Heiligenstadt (endstation of the 38A bus) out to Durnstein. You will switch trains in Krems, a wonderful town in its own right that is certainly worth exploring a bit. But go on to Durnstein, a tiny hamlet on the Donau that has a wonderful ruin above. The church in Durnstein (where Dick & Dagmar were married) is facinating, full of mumified bodies on display. The ruin is a very nice (but very vertical) hike with a spectacular view from the top. Imagine the age of castles. This is the place Richard the Lionhearted was imprisoned while Robin Hood challenged the Sheriff of Nottingham.
Visit the Prater
If you like theme parks for state fairs, then the Prater might be for you. It takes a few hours to explore, so save it for a day or half day. Its on the U2.
Visit Tiergarten Schönbrunn
If you like zoos you can see how Vienna does a zoo. Again, this also takes a half day or so, and is only worth doing on a longer visit. Schönbrunn is beautiful, though, and worth a walk around before or after the visit to the zoo. We also love the mazes in the garden, especially fun with small kids.