Actually... I think what I did worked out well... and would suggest it to other Spirit Airline travelers.
I stayed at the Mansion Teodolinda hotel, which is about 3 to 4 blocks from the Crown Plaza hotel and the Colonial Mall complex. They provided transportation to and from the airport for $7.50. They also provide courtesy or "free" transportation to nearby places, like the Puerto Salvador Allende (also known as the Malecon - or board walk / shoreline), local shopping plaza, etc. They provide affordable taxi service (much safer) which is priced better than taking one off the street (you just need a cell phone to call them to retrieve you).
If I had only a day there, I would eat their free breakfast (good), take their courtesy "city" tour, which includes local places like the home of the former dictator, parks, old cathedral, etc - so you know your way around. Then walk from the hotel to the Malecon (about a 20 minute walk if it is cool in the morning - I would not do it at night). As a gringo, they will pester you to buy pop, ice cream, etc - but just keep on walking, or say no gracias. They do it to every one that looked like a tourist. Along the walk, you will see a giant metal facial profile of Hugo Chavez, many large yellow metal trees (all will be lit up at night), then to another park with tall memorial of fallen revolutionaries, and the husband of the first female president who was killed because he was a journalist, then walk down the yellow tree boulevard... until you reach the mail street (which I think is the pan american highway). There you will see some large bronze statures of the worker/revolutionary. Cross the street where you see a large pentacostal church and continue to the park that adjoin the old cathedral, the national theater, and the cultural center. More monuments to revolutionaries, and I guess the founder of the sandanista. Afterward, head toward the malecon, or the lake side park. You will see an open air national theater across from the John Paul the second memorial and park. Avoid the first stretch of the malecon, as it is dangerous, and head toward the entrance to the Puerto Salvador Allende part - which is safe, and where most people spend their weekends with the family. You can probably spend 1 to 4 hours here, eating, and taking a tour of lake managua on a boat. Unfortunately, the tour boat and the cultural center/museum is not open or running on mondays!
I think this is a great place to spend the short morning, and short day in Managua. Going to Laguna de Apoyo... I am not sure... Maybe... but it is too long of a trip... and then heading back to the airport to pick up my friend?? Not sure... The monkey hut was fun.. but walking down that dark hill and path at night? I am not sure... I would save that for a day trip.