Time is the question, how much time?
Rainy season is at its peak...so be prepared.
Though using a rental in CR makes sense, consider using local busses, shuttles, and taxi hire in Nica. No two venues are more than 2 hours apart, and you dont want drive in Managua, makes San Jose like a country club. Roads are poorly marked as well, and there are recent reports of tourist getting shook-down for minor legal issues in order to get bribed. You have been warned.
Nicaragua is great contrast to Costa Rica, and the colonial charm of Granada is worth at least 2 nights. Ometepe is a really slow pace and detached from the everyday politics and feel of mainland, allow 3 nights if possible. Its also a lot easier to get there without a car on the ferry, and the roads are almost mud paths on the island, no fun.
If you loved the caribe feel of CR, which I do to, then you have to fly to Corn Islands at the end, via Managua. Allow 3 nights there for sure, its rustic, raw and another culture in itself.
Leon is the cultural bedrock of all things Nica, with the University as the epicenter, and lots of Arts from dance to poetry to murals and museums, at least 2 nights there. The Volcanoes nearby and to the north are lovely, a cool respite, as its mostly low land and inland in Nica, and man, Leon and Granada are HOT, like really HOT! Try to get a hotel with a pool in Granada :)
Moon Nicaragua Guidebook, 2nd edition, is great.
Enjoy, Nicaragua has some of the most humble, good hearted, hard working, salt of the earth folks you could ever meet. If possible, hit Massaya (fun) market for donations to a Orphanage in Granada. $50 goes a long way.