Make sure to keep your identity papers on your person at all times, and remember that you may have to pay a $42 departure tax when you leave. If you’ve been visiting one of the other CA-4 countries (Guatemala, El Salvador or Honduras), you can travel to Nicaragua without immigration formalities. Check out the U.S. State Department webpage for more info.
A U.S. passport is required to enter Nicaragua, and it's important that the passport is valid for the duration of your stay. You'll also need evidence of a valid return ticket or enough money to support yourself for the length of your trip. Even though you won't need a visa if you're a U.S. citizen, you'll have to pay $10 upon arrival for a tourist card. If you decide to stay longer than 90 days, make sure you obtain an extension from Nicaragua's Immigration department in order to avoid a $2 daily fee.