Middelheim Museum is an outdoor sculpture park not far from Antwerp city. Entry is free, but note that it is closed on Mondays.
You could spend a whole day or more exploring the art in the park, there is genuinely ‘something for everyone’ from hyper-modernist work, to old school traditional sculpture.
The 21 or 32 from Berchem Station will get you there, or even better go on a bike. This is Belgium so there is of course a cafe in the grounds. Stop for a borreltje of jenever and a cheesy snack.