I love Brussels - 3 trips in 5 years! You've picked a great time of year as it's far nicer in the summer than in February when it was so cold that you needed to buy tights (hose) to wear under your jeans!!

You can buy a 3 day tram/bus/metro pass for just €9 which gets you around the city with ease

I would highly recommend the Lonely Planet Best of Brussels guidebook for a short trip.

Some places we've enjoyed are

Drinks at the Grand Place late in the evening when the light show begins - so atmospheric

A tour of the Cantillon brewery

Incredible biscuits & cookies from Dandoy

Chocolate from Planete Chocolate

A relaxed meal at L'Ultime Atome which is a 5-10 minute walk from your hotel - great food & a spectacular beer list

Lunch or dinner at my favourite Belgian restaurant int'spinnekopke.
They have a set lunch menu Mon-Fri for about €13 - amazing value