Check out the Volcan Arenal (an active volcano) when you drive back to San Jose from the Nicoya Peninsula or the Guanacaste region.
It should be the dry season, so you can watch the volcano erupt at night. (If you go in the wet season, clouds may obscure the volcano for days at a time.)
http://www.arenal.net/
Costa Rica isn't really a diving destination, unlike Honduras or Belize.
Have fun.