I am both American and Belgian so know each major city in Belgium very well. I think you have to see Antwerpen, Gent and Brugge. While Brussel isn't exactly the best city to visit, there is a decent amount to see. I would highly suggest not seeing Mannekin Pis. He is tiny. Huge disappointment. I would suggest seeing the Grand Place. If you guys are interested in music and/or comic strips there are museums that are fairly close to the Grand Place. Here is what I would do in your position:

June 23rd- Spend pretty much all day in Brussel. You might want to take the Hop on Hop off that leaves from Centraal Station if I remember correctly( 2005 was last time I took it). Do one round to see the sites. Go to Grand Place, take a lot of pictures. Visit museum or two, then visit one of the parks that Brussel has etc. Early Evening, get Wafel for a leisure walk. Then go eat dinner but not near Grand Place since everything is jacked up. See Grand Place at night then go back to the hotel.
June 24th and 25th- Go to Gent and Brugge. Doesn't matter which day you pick. They are in the other direction so get those out of the day.
June 26th- Antwerpen.
June 27th- Maybe do Waterloo but not sure if there is really anything really to see there other than a Lion. At least it is fairly close to Brussel.