We will be in Vienna for two nights (one full day). Any suggestions as to what to see in that one full day? We have reservations to see the Vienna Boys Choir on the 2nd night.

Schönbrunn, the Schatzkammer, and/or one of the major museums, depending on which art period interests you.

Was just there and will warn you that the first floor of the Belvedere, with the Klimts and other similar work, was closed for some period of time. So check if that if "The Kiss" is on your list. Kunsthistorisches Museum is an excellent art museum.

Also the Lippizaner tour and the palace rooms at the Hofburg. And I was surprised by the zoo at Schonbrunn, which was much nicer than I expected.

I would get going fairly early and take a tram around the ring; that will give you a quick look at some of the main sights. Then I would go for a stroll along the Karntnerstrasse and the Graben. Afterwards, I would stop for a light, quick lunch. Then off to one museum four a couple of hours (hard to choose--if you're really keen to see the Klimts, go for the Belvedere, otherwise pick the Kunsthistorische, Hofburg or Liechtenstein museums. Or maybe the Hundterwasser Haus if you'd rather something more contemporary and eccentric). Then a coffee and pastry break at one of Vienna's many oustanding cafes (if you go to the Kunsthistorische museum, they have a nice cafe serving pastries from Gerstner's).
If, on that first night, you get in early enough to have dinner, I would suggest Meinl am Graben, terrific food, reservations necessary and try to book a table overlooking the Graben. If you will be there in the winter, after dinner (if it's not too late), go over to the Rathaus to watch the skaters and have some punch. Note: you are welcome to join the skaters, but be advised, the skates they rent aren't sharpened that often.
Unfortunately, if this is your first trip and you only have one full day, I'd skip Schonbrunn. Lovely, but too time-consuming.

I agree with basically everything BTilke said with regard to the sightseeing. The ride around the ring road is a great introduction into the city and I highly recommend it. I would also include a visit to St. Stephan's Cathedral; its right on Graben and its worth going into.

Tracy

Wouldn't bother with Meinls for food - try
For traditional Viennese
König von Ungarn
Drei Husaren

For Styrian
Steirereck

For the setting
Palmenhaus

If you do go to the Schönbrunn, dine at the Altwienerhof

Lawchick, *you* are free to not bother with Meinl, but other Fodorites who recently dined there loved it. As do Gault-Millau, which gave it 3 toques and a 17 rating (just a shade behind Steirereck; Drei Husaren didn't even make the rating 13 cut-off) and the leading Austrian wine/gourmet magazine named it Restaurant of the Year in 2005. One of Vienna's newspapers also named it the number one restaurant in town, ahead of Steirereck im Stadtpark. Apparently many food and wine critics believe it's a restaurant certainly worth a bother.

I guess it's kind of like the "Should I bother going to Madrid" type question. Some like Meinl some do not. The tourists I've shown around Vienna have prefered the more traditional type restuarants. It's only my opinion, it's here to be taken or left, or indeed trashed, which seems to be the recent habit unfortunately of BTilke.

If you like to see crown jewels (Hapsburg dynasty), go to the Treasury (near the Kunsthistorisches)