This northern section is the main target for most visitors to Rwanda as this is the home of the mountain gorillas.
The protected area that contains the gorillas is spread across three countries ... in Rwanda it is the Parc National de Volcans, whilst on the Uganda side is Mgahinga National Park. The DRC portion remains off limits for political reasons.
The best base on this side of the mountains is undobutedly
Virunga Lodge, with
Gorilla's Nest serving as a much simpler and lower cost alternative.
Be warned ... this whole gorilla thing is unavoidably expensive.