Depending on your plans in Brussels, Villo might be suitable for you as well. It is a public bicycle system, similar to Velib in Paris, where all over Brussels bicycles are available for you to use. The idea is simple once subscribed you can take bicycle in one of the racks and cycle around for free initially. After 30 minutes of cycling, you'll need to pay an additional fee. However, you can have as many 30 minutes slots as you like while exploring Brussels, so with some planning you can explore Brussels quite cheaply. You don't need to return the bicycle in the same rack, which enables you to cycle from point to point.
You'll pay 1,60 per day or 7,65 per week for a ticket and if you manage to cycle in portions of 30 minutes that is all you pay.
Personally, I prefer the cycling over taking the bus and/or the Metro, which in Brussels can be confusing at times.
The Villo system is only available inside the center of Brussels. You will not be able to use the bicycles from neither of the two airports.