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.
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hey!! I recently visited San Jose and is a very safe and quiet city. I’m very satisfied with this visit, it is interesting to see the progress we have in Latinamerica, I have visited other countries in that part of the continent and Costa Rica is really the one that i love. Is very nice to go to the beaches, which are just 30 minutes from the city and are beautiful and very clean. Of course visit the Poas and Arenal volcanoes, which are active volcanoes. As for the stay I had the chance to stay at the Intercontinental and it seemed a good choice, the hotel is big, the atmosphere is friendly and has the biggest mall just in front of it! which is ideal to go shopping… Here I send more info in case you want to research:
http://es.centralamerica-photo.com/volcano-poas.php
http://es.wikipedia.org/wiki/Arenal_%28volc%C3%A1n%29
http://www.realhotelsandresorts.com/rhr/info/es/RICostaRica.aspx