What would you do if you had one day in Brussels?

I bet Rick has a whole itinerary all neatly planned in his book ...

I just left there this morning.
Take a walking tour, perhaps.

Depending on the day, visit the Old Masters museum or visit Waterloo. Eat fries; drink beer.

I'd spend the day in Bruges since it's only an hour away.

See the Grand Place, have lunch there at the Roy d'Espagne (food is nothing special, but the view is great), then either do a guided tour or find one online of the art nouveau houses south of the centre, finishing at the Horta Museum.

I would spend the entire day in Little Delerium.

You could probably spend you time just wandering central Brussels. The Grand Place, the surrounding streets. There are no real "must see" museums, but there are some quality museums depending on your interest, those you would need to look through on line based on your interest.

If beer is of interest, there are a number of good beer bars where you can sample a range of Belgian beers. There is also Cantillon Brewery if you hit the timing right, a neat old brewery producing lambics (a sour beer). There are of course a few craft breweries.

Several have mentioned going elsewhere, but for what you want, just stay in Brussels, you will see Ghent and Bruges, I suppose if you find nothing of interest, a daytrip to Antwerp would be the most logical choice.

In Brussels the Comic Art Museum, Magritte Museum, Old Masters Museum as already noticed, Royal Fine Art Museums and the Musical Instruments Museum. All within walking distance from the Grand Place or Brussels Central railway station. The Natural Science Museum with it’s Dinosaur Gallery, Royal Military Museum, Art and History Museum and Autoworld needing a serious walk or the metro is much easier ofcourse.

South of Brussels the Africa museum in Tervuren. As already mentioned Waterloo and to combine with the Wellington museum.

You don’t ask for daytrips outside Brussels, nevertheless remains an option too ofcourse.

Since your tour goes to several of the day trip locations people have suggested above, I would just stay in Brussels to get over jet lag. I spent just afternoon in Brussels (not before a tour) and this is what we did: walked around the historic center/Grand Place, walked to see Manneken Pis, had frites and waffles, went to Comic Book Museum (because we were there on a Monday and most everything else was closed). In hindsight, a walking tour would have been more informative but it was easy to get around on foot.

Day of the week might involve museum closures. What month of the year? Also consider the cultural and ethnographic displays in the Bozar museum. Belgian history at the City museum, and the unusual Coudenburg Palace "cellars" museum.The Mannekin Pis statue is not important to see, except that people will ask you about it. Your personal interests make a big difference, especially in museum choice. The Ancient and Modern art museums (unified) could take up a full day for a serious art fan. Don't waste any time on the European Parliament or the Atomium. Start at Grand Place, and walk through the historic shopping arcade (I forget the name), up to the art museums. You'll pass some nice churches and historic parks. I like a glimpse at the Rue des Bouchers restaurants, but they have a bad reputation for dishonest (unwanted) plate serving charges and the like, with pushy touts at each restaurant entrance.

If you are fanatic about Peter Paul Rubens or James Ensor, you might take the five trains per hour to Antwerp, for the world-class KMSKA Museum, and the popular MAS museum (not adjacent to each other, however.)

I have not been on a Rick Steves tour.

I’d go to Antwerp, Ghent, or Bruges. Other than the Grand Place I don’t find Brussels interesting.

I would agree with jaimeelsabio

With one full day in Brussels and decent weather, I would take public transport to Waterloo and try to see the museums, shops, and the town pertaining to the battle. Much more detailed and expanded now since the bi-centennial than when I visited the place in August of 1984.

As a grad student currently living in Brussels... here's some things you could do to take up a day:
morning visit to the flea market on Place du Jeu de Balle
Grand Place, obviously. get chocolate at Galler nearby
Magritte Museum
Sewer Museum has been a big hit
if nice out, walk through one or more of the parks ... Park Cinquantenaire, or Bois de la Cambre if you have the time
get a drink at Cafe Belga or Le Pantin off Flagey (try the Belgian lambic beer, kriek)
fries at Frites Flagey or Maison Antoine - get the andalouse sauce

If the weather is good, there are several attractive exterior visits, not just Grand Place, but then retrace your steps to Petit Sablon and Mont des Arte, and via Galleries Royale to the unified Ancient and Modern Arts Museums, for paintings like "The Death of Marat." For those who don't really like paintings, there's a Horta Museum, or the Ethnographic and Cultural museum Bozar - which right now has a special James Ensor exhibition. Several of the smaller churches are even more attractive inside than the Cathedral.