Granada
The colonial town Granada is located only about 1 hour from San Juan del Sur. It offers great sightseeing (lake, small islands in the lake, volcanoes, colonial structured houses, museums, etc)

Masaya
It is another good city to see, there is a market there where you can find any kind of local hand made souvenirs.

Popoyo and the beaches up north
If you are a experienced surfer and want to hit some massive, bigger and powerful waves, make sure you take a trip up north to the beaches near by Tola (1.5 hour from SJDS).