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Nice gardens to visit
Here are plenty:
http://discoveringbelgium.com/?s=gardens
When the roses are out there is the stunning rose garden in Sint Pieters Leeuw.
Also take a ride out to Hoegaarden. You enter right on the main square and it's lovely and very surprising. The café is good too and I think there is a kids play area too.
On the subject of kids' play areas and gardens, go to the Public Domein at Huizingen.
http://www.xpats.com/10-ways-fill-your-urban-green-lung
check out the one on parks!