Learn Spanish -
Know the basics. You can get by with very little Spanish. You should learn the basic phrases so that you can ask questions. You probably will not understand the responses and they may or may not be accurate but at least you try. I suggest a lot of gesticulation. Practice at home by playing charades.
Don't expect street addresses - Few streets have names and even fewer houses are numbered in Nicaragua's towns and cities. Finding something can be a real adventure. Maps are only accurate sometimes and things often change. Embrace the crazy and just enjoy your day out searching. Try to stay on the shady side of the street. Also, expect to be told to go to where a landmark used to be and head towards the lake.
I suggest to everyone to travel during the rainy season in Nicaragua. Unlike some of the neighboring countries, it does not rain heavily every day. The land is brilliant and full of life during the months of late May- mid-November The rain comes hard and leaves quick. It is very common that it only rains at night. The occasional tropical storm comes through and you have a few days of rain but it is warm and nice. It is very uncommon for travelers to melt in the rain so do not worry about getting wet.
There is a great book called, Blood of Brothers which offers a great first-hand account of Nicaragua's brutal history in the 70s and 80s. It is a great read and will help you understand why some of the things are the way they are. For instance, why does everything come in a plastic bag? Also, it is good to read guide books to get an understanding of what is out there for you to explore. Do not take them as gospel. Things change in Nicaragua fast, sometimes faster than the books can update. Treehouse Poste Rojo is not even in the books yet.
Bring Gifts -
Everyone loves the gift bearer. tee shirts, clothing, nick knacks, frisbees, kids toys and used baseball equipment are great to have when you want to befriend the locals. I often buy marbles in the market and carry them in my pocket. It is fun to hand them out to random kids and watch them smile. Of course, they will probably be used for sling shot ammunition and shot at some poor bird but try to think of the smile not the bird.
Stuff to Bring -
A flashlight or head lamp, a day pack and a larger pack. I prefer to pack all my clothes only in my backpack. If it is lost it is only replaceable items. I always keep my day pack on me. I carry my passport, camera, credit cards, money, phone and other valuables in it and it never leaves my site. You might want insect repellent and sunscreen. They are expensive in Nicaragua.
Passport Copy -
I email myself a scanned copy of my passport to a secure email server so that I can print a copy anywhere there is Internet access.