Well, since I used to live in Nicaragua, I can give you all the information that only the locals know. For instance in Managua, if you want to go to one of the mercados (markets) be sure that you don't go alone. Sometimes the mercados can be dangerous if you go alone. I suggest going with a group of 3. Now, if you want to go to a good mercado that is relatively safe, then you should go to the Mercado Huembes. It is large and has a large selection of pottery, food and other goods for very cheap prices. You should always try to barter as the sellers usually will go down from their original price. One thing I don't recommend doing: try not to go to the Mercado Oriental as it can be very dangerous for tourists.
Now, there are many other things that you can do in Managua. You can go to the local nightclubs, a very good club is Hipa Hipa. You can go to the malls in Metrocentro, Plaza Inter and Galerias Santo Domingo. You can also go to the Palacio Nacional and catch up with the history of Nicaragua. Managua has many things to offer if you look closely and enjoy the flavor of it.