What sets the itinerary
Two activities are permit-constrained and everything else bends around them: gorilla trekking, where permits are limited and dated, and to a lesser degree chimpanzee trekking. Fix those first, then build.
The rest divides by effort. Hard: Karisimbi over two days, Bisoke in one, chimpanzee tracking. Moderate: the Fossey tomb hike, golden monkeys, colobus, the longer Nyungwe trails. Easy: game drives, the Lake Ihema boat, birdwatching and the Kigali city tour.