Guatemala City and Managua is fine. If on a longer layover in Guatemala City take a taxi to Antigua and explore the city. Amazing place and lots of great places to eat. In Managua you can tour the famous Chatedral or go ziplining. Tired of hearing how unsafe it is. Be smart, like hopefully one would be in any bigger city or unfamiliar area. Bring an id, a few dollars, a credid card and your phone. Enjoy the layovers.

POS is fine as well if you want to explore during the day.