Antwerp - train + aquarium

just go to any train station and ask for the leaflet/brochure "B-Excursion"; it is quite self explanatory and it shows all the attraction in Belgium and surrounding countries (UK- France - NL) that can be reached by train; and that includes your trip to Antwerp. When you buy the ticket just say that you want a B-Excursion

I took some of these and it was quite a nice day out.

you can also find info online,