Face it; if you are going to visit Brussels, you're more than likely going to get wet. It rains quite a bit in the whole country, as a matter of fact. Don't let that put you off enjoying yourself though. When it rains in the main square in the evening and the lights are on, it makes for some dramatic photos with the lights and buildings reflecting on the wet cobblestones.
No matter how many times I stroll through Brussels' Grand Place, I always have to stop and admire the architecture, and also snap some photos. And I must have walked through that square hundreds of times in my life.
For interesting photos when the sky is dark and you don't feel like using a flash (which isn't very good for the background) switch to black and white!
And of course stop by the Godiva chocolate shop or beer museum or just one of the cafes for some refreshments.