Another outdoor activity near Brussels on a beautiful day would be the Lac de Genval. You can walk around the lake in about one hour, then sit back, relax, eat & drink something in one of the restaurants. Recommended is Genval.Les.Bains at Hotel Château du Lac.

Yes, that's also a nice idea. It's on the same train line that serves La Hulpe (Genval is one stop after La Hulpe). When I worked in Genval, I used to enjoy riding the local train with the lovely, old-fashioned self-striped, high backed plush velvet seats in first class.
The easy walk from the Genval train station to the hotel and the lake winds through a residential area past some interesting homes.

Of course, Strasburg is too far. I meant to say Cologne, but it looks like you already made up your mind.

tentek: thanks! cologne is something we'd like to save for when we have more time, since we have friends there who we'd like to visit.