This information is posted on the US Department of State Travel Page.

To enter Nicaragua as a tourist, all you need to do is purchase a $5 tourist card upon arrival. Technically your passport also needs to be valid for the length of your stay in Nicaragua, not the six months that it does with most other countries. You also must have proof of travel onward, either back to your country of origin or to another country, and evidence of sufficient funds to support yourself while you are in Nicaragua. The funds aren’t exactly something you can prove at the airport, so they don’t particularly check for it, but you should make sure you have a ticket of travel onward, even if you plan on renewing your tourist card, or applying for residency once in Nicaragua.

The tourist card that is issued is usually valid for 30 – 90 days. If you wish to stay for 90 days, request it right there on the immigration form given to you  most likely by your airline. If you don’t see the box asking for how long you want the visa for, make sure you ask the person admitting you to the country that you wish to get a 90 day visa. The cost is still the same – $5.

A departure tax of $32 is also charged, but most airlines include that in the price of the ticket. 

Nicaraguan law stipulates that foreigners must always be in possession of a valid identity document while in Nicaragua. A valid passport with your entry stamp is acceptable, however, it is not advisable that you carry that with you. Instead, make a copy of the photo page and the page with your stamp on it. Carry that with you.