I love Waterloo having lived there for 14 years, but not that practical for a day trip with the other options available. The main sights of Waterloo are not associated with WW1/WW2 but more Napoleonic times. Also the battlefields are actually nearer Braine L'Alleud station if you did decide to train it there.
If you headed by car to Luxembourg you could take in the Ardennes, home to Bastogne and the Battle of the Bulge WW2 sites.
Leave plenty of time for Bruges..maybe an overnight? Also Leuven is an underrated place.
Enjoy Belgium, home to the worlds best Chocolate,Beer and Fries (frites).