I know that part of the world rather well, so, for what's it's worth:
Don't be afraid of driving; you wont do much without a car.
Interest in history? Waterloo is a hour from downtown; if Nappy had won, world would be much different. WWI major battlefields are a couple of hours by car. Crecy (Edward III), Agincourt (Henry V) are couple of days rt and can be combined with WWI. South and a bit east WWII Battle of Bulge in beautiful Ardennes Forest and north are Bridge Too Far
sites. Aachen, Charlemagne's hq (780's-815 AD) a day trip.
R&R? Great hotels (see SRS web site) on Channel coast. No good in winter but wonderful in summer and fall.
Day trip to Brugge, a gem. Couple of days in Holland, towns much nicer than cities, delightful everywhere.
Paris, can be done as day trip using swift train.
Again for what it's worth: use a car, don't stay in Brussels as there are literally dozens of very fine small hotels all over the nearby country. If you want specific hotel suggestions, ask and I will email more to you. Roads in Benelux are good, well marked, easy to follow, not crowded except in cities and on commercial routes to the ports. Be courageous; you will not regret it nor will your dad. I've got a few years to go before I equal his age and I hope my miles don't expire; if I don't, they wont.
Cheers,