Sorry to hear about the heat. Some things that I have done:

- Go for a swim... ideally in the ocean or a river. Open air pools often have cool water as well due to cooling at night (if it is, in fact, cool at night). In the absence of a swimming area, just take a cold shower. It has a similar effect, but not quite as thorough at cooling the body down.

- In general, if you are not on the coast, get to the coast. The ocean water usually keeps the temperatures much more moderate. Even as little as a few kilometers can make a huge difference.

- Go spelunking. Caves maintain a fairly consistent (and cool) temperature throughout the day.

- Go to a winery, ideally one at which you can spend some time in the wine cellar and/or chai. Some beer breweries may have similar facilities.

- Go to a movie theater. I am not sure about Belgium, but they frequently have air-conditioning (at least in the US, where I am from).

- Try to stay away from carbs when you eat.

- Ice packs on the back of your neck does wonders.

Good luck!

Edit: Spelling.