Walking
Most tourist places are within walking distance and doing everything on foot will definitely get you some exercise.
Nice parks to take a stroll in are

- Brussels Park (in front of the Royal Palace)
- the Cinquantenaire Park (where you may find yourself jogging around the perimeter track with young recruits from the  Royal Military Academy on Avenue de la Renaissance)
- the Parc de la Woluwe and Etangs Mellaerts (not far from the Montgomery metro station or take tram 39/44 from there to the Chien Vert stop)

and a little further away
- the Parc Josaphat
- the Parc of Laeken (where you can also visit the Japanese Tower and the Chinese Pavilion) NB: The Royal domain is closed to the public except for the annual opening of the Royal Greenhouses.
- the Bois de la Cambre
- the huge Foret de Soignes at the South of Brussels: access at many points on the 44 tram route or from Rouge Cloître (near  Hermann Debroux or the St. Anna terminus of the 96 bus)

For a nice 5km+ walk/run/bikeride where you can't (easily!) get lost, take the metro to the terminus at Stockel, then walk to the back of the train and up the steps - follow the pathway and it will bring you onto the "Promenade du Chemin de Fer", which replaces an old railway line. This crosses over  the Avenue de Tervueren (where you could also start or finish your walk - 39/44 tram stop is near the Tram Museum), continues parallel to the Boulevard du Souverain, above and alongside the Parc de la Woluwe, crosses the Chaussée de Wavre, and ends near Demey metro station. When you come down off the track, turn left and in about 3 minutes you will come to Demey.

  
Running
Nice and green places to run are the Bois de la Cambre (closed to traffic every Sunday) and the very large Foret de Soignes, at the South of Brussels. Every year in May, you can also participate in the 20km of Brussels which starts and ends at the Cinquantenaire, and make your run across many different parts of the city. It is a very popular event, with people of all ages and levels participating, and a lot of animation along the way. For more info, visit: 20km of Brussels 

Cycling
Provelo ( www.provelo.be) organizes thematic guided cycling tours of Brussels (Art Nouveau, Comic strips and cafes, castles and abbeys, city and gardens, etc.) from April through October (and throughout the year for groups). You can also rent a bike all year round from their main location at rue de Londres 15 Londenstraat (near Trone/Troon Metro Station and Luxemburg train station)

Rollerblading
Every Friday night from early June to late September, a roller parade covering about 20 km is organized in Brussels. It starts at 7PM from Place Poelaert (in front of the Palace of Justice) and ends at 11PM. Streets are closed to traffic along the way. Participation and parking are free but you have to bring your own gear (including safety gear) and you need to know how to brake! You can rent gear from TWIN at 414b, Chaussee d’Alsemberg.
For those who are new to rollerblading, you can learn every Saturday and Sunday afternoon of the summer at the “Roller School” in the Bois de la Cambre. For more info, visit: www.rollerparade.be.

Beach Sports
From late July to late August, an urban beach unfolds at Bruxelles-les-Bains (Metro Station: Yser). Entrance is free and various fun sports are offered on specific days (beach volley, beach soccer, petanque, American Football). Check the calendar at : Bruxelles Les Bains

Spectator sports

Soccer – watch a game of local team Anderlecht, one of the leading teams of the Belgian Championship ( RSCA). If you are lucky, you might be in Brussels when the Belgian national team (the “Diables Rouges/Rode Duivels”) plays.

 Athletics Track & Field - Memorial Ivo Van Damme – every year late August

 Running - 20km of Brussels