A restaurant I highly suggest eating at in the capital of Managua is Cocina de Doña Haydee. It's my favorite restaurant (probably in the world). It's fantastic traditional Nica food with queso frito and tajadas verdes, which you must try. The cost is inexpensive compared to US prices.
Eat quesillo while you're in Nicaragua. It's a tortilla with cheese called quesillo, a sour cream, and onions. There's a place right outside Managua on Carretera Masaya called Mi Viejo Ranchito that sells great quesillo.
Casa del Cafe is a little chain in Managua (and maybe elsewhere but I don't know) that has empanadas, coffee, cakes and sweets among other things. I usually order the empanadas. The service is good and food is inexpensive.
There is a tasty little Taquito stand around Km 12.5 Carretera Masaya. I usually order the beef taquitos (3 flour tortillas with beef, onion, cilantro, and a lime) with a bottle of Coca Cola. Delicous!