Unless you have some very odd airport arrival and departure times, I would still recommend investing only a bit more time, effort and expense and base yourself in Granada for the most of the 24 hours. You are talking too short of time of course to really do things and it depends on how much money you feel comfortable spending, but you could visit the San Francisco, Church and Convent Museum for what little Nicaragua has been able to keep in pre-Colombian statues (some have liked visiting the church catacombs, which I have not done-check visiting hours), and other churches in town, eat well enough in various palaces there and just hang out around the plaza. If money is no issue, hire a recommended cab to go back to the airport via Masaya market and Vulcan Masaya park to look inside the crater.