There are lots of activities to do while in Granada and the most popular ones are touring the city centre, taking a boat ride around Las Isletas and visiting the volcanoes (Mombacho and Masaya). A few other cool things you can do in Granada are watching pottery demonstrations in San Juan de Oriente and visiting the white villages.
The volcanic waters are said to have healing powers so take a deep if you can. Parque Central is a nice place to people watch and buy miscellaneous stuff from the street vendors. While visiting Mombacho volcanoe be sure to also do a zip line tour. It gives you a nice bird's eye view of the area.
if you go Cerro Negro volcano, one cool thing to do there is volcano boarding. Once you climb the volcano you can board down the other side on a stand-up board or sit down like a sled, which is what I did. It’s great fun!