First of all, try Pro Velo. You can rent all sorts of bikes and accessories for periods ranging from one hour to four weeks: city bikes (15€/day), 2-person bikes, bikes with training wheels, extra wheels, child seats, carriers for kids. These rentals are great to use at your leisure.
At the start of the Forêt de Soignes to a riding tour of the Promenade Verte and Parc Tournay-Solvay, Km 10 rents bikes (25€/day) and especially 2-person bikes (50€/day). This atypical machine transforms an individual sport into one you can share with friends. You'll need a little bit of coordination to ride the tandem bike.
However, stick to two wheels when using a bike as a mode of transport, for example as an alternative to the car or to complement public transport. The new Villo system, organized by the Brussels City authorities, is perfect for such cases. Using either a subscription or credit card, you can use one of the 250 bikes parked at 23 stations 24/7, which are conveniently located only about 300 meters apart! The advantage is that you can pick-up the bike at one station and then drop it off at another... This really practical system would be even better if it covered all 19 communes.