I am not convinced it will be quite hot or dry enough for a picnic - not on the grass at least - so suggest we meet at au laboureur in the Saint Catherine district.
Nearest metro Saint Catherine, just a short walk from the Bourse and/or De Brouckère metro too.
Make sure you have worked out how to get there before setting out, it is a little bit off the beaten track.

http://www.ebru.be/Cafes/cafe-bruxelles-1000-laboureur.html

Address:
Au Laboureur
Rue de Flandre 108
1000 Bruxelles

With any luck we will be sitting outside - there is plenty of outside seating.

For those who haven't met me yet you should be able to recognise me from my profile photo.

So lets meet there as of 7pm and if people then want to go on and eat there are plenty of restaurants nearby.
Looking forward to a relaxing end of the week drink with you all!!
Catherine