In Nicaragua, check out Lake Granada and San Juan del Sur.
In Costa Rica, check out the Bahia de Salinas (this may be the best spot in Central America by far), Tamarindo/Playa Grande/Playa Langosta (long stretch of beaches with a great party town), and Lake Arenal.
If driving through Central America, make sure you ahve a copy of the title to your car. Leased, financed, and rental cars are not allowed to travel between Honduras, Nicaragua, Costa Rica, and Panama. Weird legal issues, but I have been turned away at border crossings due to it. THis is a recent issue as years ago it was possible to do this.