There really isn't anything in San Juan del Sur worth stahying in town to see. I'm not a big fan of long excursions, but the trip to Granada was well worth the time and the money. You can see none of the grandeur of the landscapes and volcanoes in the port town.
We took the "Colonial Granada & Isletas Boat Ride with Lunch" tour and it was one of the highlights of the cruise. The motorcoach ride is long but the scenery is beautiful. The boat ride among the tiny islands on Lake Nicaragua was wonderful; spectacular vistas of the volcanoes; lush vegetation; lots of wildlife; and an amazing variety of homes--everything from shacks to mansions on the little islands. And the lunch was one of the best meals on the trip.