If you had 3 weeks in Nicaragua, what would you see?

I would add Granada and the Rio San Juan if you are into wildlife. If not go to Ometepe after Granada. Do day trips to Lake Apoyo, Masaya Volcano and Market , Mombachu Volcano and even to San Juan del Sur. Do a horse carriage ride around town, take a boat or kayak ride through the Islettas, climb to the church bell tower and get a view of the city go to the market place, go to the museum at the San Francisco Church. There's plenty to do.
Keep one thing in mind through all of this. December is the last month of rainy season and it can get REALLY rainy in that month. It doesn't always happen but it is not the best travel month for Nicaragua.

Thanks so much for all the information everyone!

I agree with #2, go and explore the northwestern coastline of NIcaragua . If you want quiet beaches, no one to harass you or bother with the large party scene, that's where you want to go. Playa Jiquilillo, Aposentillo, Mechapa, and Asserradorres are gems and there are very little tourists compared to San Juan Del Sur, Riva's Tola area, and Leon's Las Penitas and Poneloya. If you are looking for authentic Nica fishing villages with quiet beaches, empty waves for beginner and intermediate surf, you've hit the spot. Not to mention Playa Jiquilillo has 7km of empty beach and is also home to the largest wetlands in Central America. Reserva Natural Padre Ramos is a must for birdwatching, kayaking, and exploring the mangroves. Volvan Cosiguina is also located nearby. For lots of info about the surrounding areas, check out Hostel Rancho Esperanza's website at www.rancho-esperanza.com, ibis kayaking for the wetlands www.ibiskayaking.com, and Redwood Beach Resort's page for Mechapa. If you plan on traveling north toward El Salvador (especially surfers) definitely check out these areas because there are empty waves to surf and you can head north to Potosi and take a boat across the Golf of Fonseca and enter El Salvador via La Union and you have only an hour to hit some prime surf in El Salv.

Granada. Ometepe. Rio San Juan. Corn Islands, Laguna de Perlas. If time: Selva negra /matagalpa.