Kigali is the capital of Rwanda and its most important city. It’s mainly an administrative centre and even before the civil war in Rwanda there wasn’t really a lot to do and see.
There is a crafts market in the centre of the city, as well as a Muslim Quarter replete with a nice mosque.
In the city, you can visit the genocide memorial and the Hotel Milles Collines, places relating to the 1994 War. There are a few other memorials in the city as well.
It is a useful place to arrange stuff for a trip to the Virunga Volcanoes. The ORTPN is the place to go to arrange such excursions. For most national parks it makes sense to use their services because tourist infrastructure is not always readily available.