The space centre in Kourou is the location of the European Space program. If you are lucky, you can even watch the take off of an Ariane rocket, as it happens about once every three weeks. The best way to get tickets (free) to a close location (the closest is at 4.5km) is to go to the visitors center a day or two before the launch and just ask for them.
To watch the rocket taking off is incredible and it is very strange to see the launch and not to hear anything, as the sound takes 15 seconds to reach you, while the light (your vision) reaches you right away.
It is also possible to visit the centre on a tour (also free) and to see the different launch pad and facilities. A good museum of space also displays everything you want to know about the space program and is really worth seeing.
Overall, if you are in French Guyana, it is a must to visit the space center.