If this is your first time in Belgium, I would suggest looking into a day trip to Bruges rather than spending the day in Brussels. Bruges is easily reached by a short train ride from either Brussels or Ghent and is more charming and has more interesting sights than Brussels. The only must see in Brussels is the Grand Place in my opinion, which you can easily do for dinner or drinks at the end of the day once you get back from Ghent or Bruges. Brussels could easily fill a day however depending on your specific interests, especially if those interests include shopping, chocolate and beer. The Royal Museum of Fine Arts in Brussels is a minor art museum by European standards, but if you love art, is still worthwhile. As far as restaurants, if you choose not to eat on the Grand Place for the atmosphere, then Brussels is famous for its art nouveau cafes, including Falstaff and Le Cirio.
The big thing in Ghent is St. Bavo's Cathedral, including the Ghent Altarpiece. After that, probably the biggest attraction is just the city itself. Rick Steves has a pretty good walking tour in his book. Ghent feels similar to Bruges but is bigger and feels more like a place where people live rather than mostly a tourist destination. I still feel like Bruges might be more worth your time, but many people prefer Ghent for the reasons I described. I didn't have any food in Ghent that was so memorable that I'd recommend it, but there's a lot there.