Managua is a very big city. I didn’t find anything particularly special or noteworthy about this place but it is where the planes arrive so you pretty much start here! It is easy to get around using the buses and taxis.
Be very safety conscience in Managua. Do not walk around or near the "wrong" neighborhoods. Only take taxis at night.
There is really not much sightseeing in Managua. I loved Grenada.
I am a big fan of Nicaragua but Managua is still a dangerous and very unattractive city with little to offer.
Always travel by car or taxi and make sure you have reasonable spanish language skills. There is almost nothing to do during the day and the food is predominantly fast food outlets. Having lived iin Medellin for 3 months I guess I may be spoilt though. Real shame as the people are generally hospitable but after a 10 year break from being in Managua it could have been last week judging by the progress visible.