Things to see for day trips in Belgium

The shipping museum in Antwerp is very good. It's at the waterfront.
Or just go to the main square, get a table in front of the fountain and drink beer. :-)

Gent has some unexpected corners, such as around the Sint-Pietersabdij (Saint Peter's Abbey), and unexpected buildings which show the pride and might of their co-operative organisations, such as the Bond Moyson on the Vrijdagmarkt (Friday Market) and the adjacent trade union building.
In Brussels the area around the Sint-Katelijnekerk (Eglise Sainte-Catherine) is special. The Sint-Hubertusgalerijen (Galeries Saint-Hubert) are Brussels' equivalent of the Galleria Vittorio Emanuele in Milan. In a city which has seen a lot of architectural damage, the Koningsplein (Place Royale) is of a pleasant elegance.

The old train station in Antwerp itself is an architectural gem itself inside and out. However you could lock your bag up for a couple of hours in the train station (see info here):
http://www.belgianrail.be/en/stations-and-train/search-a-station/6/antwerpen-centraal.aspx

and head to the historic center to see Cathedral of Our Lady of the most impressive and largest Gothic cathedrals in Northern Europe, built in 1351 and the old market square which is surrounded by the typical medieval guild houses .

The historic center is about 30 minutes to walk but there are many buses
http://www.visitantwerpen.be/en/detail/public-transport

Brugge: Climb the Belfry (as long you don't have a health condition cardio)
for a great view over the city.

Thanks for all your suggestions, very helpful.

We toured both Ghent and Bruges on canal tours. We appreciated the information provided by the guides and were more confident of where to go on foot. Definitely would recommend these boat tours as the way to start your visits.

If you have any interest in World War I, you should try to get to Bruges for the Quasimodo Flanders Field tour. So interesting and informative. They picked us up at our Bruges hotel around 9 am & brought us back in late afternoon, so you would need to work out logistics from Ghent. But I think it would definitely be worth the effort.