Can't help too much on the places you are headed for. A little over a year ago we spent a bit over a week in Nicaragua - Managua, Granada, Leon, and a day+ on the beach in Las Penitas. This was around New Year's and beyond.
Managua - fantastic Christmas displays and lights in the city
We spent a couple of days each in Granada and Leon - both nice places to visit and enjoy. From Granada, took an evening trip up Volcan Masaya to see the crater and molten lava. Wish we had more time to laze on the beaches. Beaches were quite empty and very enjoyable.
Food was really excellent especially the chicken. Getting around was easy on the local buses. Only took long distance cabs twice when we didn't want to hang around waiting on the bus. Overall, prices are very cheap compared to the US. No issues with safety and no real hassles.
Your best bet for planning is to get a hold of the Lonely Planet guidebook and then decide how you want to split your time.