I visited Nicaragua's colonial cities Leon and Granada and the twin volcano island of Ometepe as part of a three month trip through Middle America and Cuba. I fount out my visit was too short for Nicaragua has much to offer in the way of adventurous travel. More volcanoes than I have seen, jungles to explore and islands to visit. The people are relaxed and the cathedrals of the once almighty church compete with signs of a once promising revolution for your attention.
Leon
The second most known Spanish colonial city in Nicaragua. Churches, colonial housing, museums and a lot of pleasant cafe's. Visit the museum of the revolution for some understanding of Latin American and Nicaraguan history. And do not forget the Fundacion Ortiz, a private museum with some interesting works displayed in a very unlikely fashion (open windows, semi outside).
The nearby volcanoes should give you a glimpse of glowing lava, but were obscured by fog when I visited. Sandboarding down the Cerro Negro is a recommended activity, although you are not likely to make the 70 km an hour the record-holders do. Don't worry, it will be fast enough.
Granada
The most known Spanish colonial city in Nicaragua. Parks, Churches, colonial housing and a lot of pleasant restaurants and cafe's. The central square is a colonial gem and the walk towards the lake gets you some nice sights. But the town is touristy, maybe a bit to much. On the other hand a must if you want to splurge on ecological and biological food. Or huge American steaks in sport bars on the other hand.
Ometepe
Relax under the twin volcano’s. But is possible to climb the Volcan Conception. Another exciting trip is circumnavigating the island on very bad roads on a motorbike. Serious offroading.