Sitting at the airport, waiting for my flight going back to Brussels, I thought it may be useful to summarise all the possible airport transfers in Brussels.
Bruxelles – Zaventem
This is the main Brussels airport and fairly easy to reach. There are busses 12 and 21 that go to Brussels Schuman or Place du Luxembourg. Depending on the traffic it takes around 30 mins. From Monday to Friday, until 8 pm: bus number 12 operates an express service, only serving the main stops. During the week, after 8 pm and all day on Saturday, Sunday and public holidays: bus number 21 operates a regular service, serving all stops. The bus costs around 4 euros.Another option is a train. The fastest one is from Bruxelles Schuman that takes 15 minutes and runs twice every hour. The train ticket is around 10 euro. Uber is unfortunately not an option as it has been disbanned from the Flemish part of Belgium where the airport is situated. Taxi costs around 25 euros.
Bruxelles Charleroi
This airport is a low cost airport of Brussels but pretty far away from the Brussels centre. A lot of Ryanair and Wizzair flights fly there instead of the Zaventem airport. The transfer to there is a bit more complicated.
If you have a car there are relatively cheap long-term parking options. Some of them you can find here:https://www.parkvia.com/nl-NL/vliegveld-parkeren/charleroi
If you do not own a car, there are charleroi shuttle busses which you can book for 14 euros one way:https://www.brussels-city-shuttle.com/en#/
The shuttle bus goes from Gare du Midi in Brussels and in front of the white tent at the Charleroi Airport.
Another option are the group taxis. In Brussels, they stand opposite the street from the shuttle busses and they wait for 8 people to fill the taxi and then they leave. At the Charleroi airport they are situated where the other taxis are. The advantage of the taxi is that it is faster as it gets to Brussels in 45 minutes rather than an hour that it takes the bus and the price is the same. However, unlike the bus that has a fixed schedule, the taxi is ready to go only when they have 8 passengers.