This is a question often asked, but rarely answered, because its very difficult to answer. There are too many variables. The chances of you just showing up as a foreigner, and finding a decent and well paying job are very low. But there are still plenty of opportunities. There are tons of people who move here and open a hostel, or a restaurant, or coffee shop, etc. Many of them fail, some succeed. Its just like everywhere else in the world. Bad service, bad management, location, financial issues all play just as much of a part as the fact its in Nicaragua. Places like SJDS, Granada, etc. are by far the most touristy, so at first glance they seem like the best places to open up a tourist oriented business. But keep in mind, that these are also the locations that you will have to compete with the other dozens of people coming down every year opening the same type of businesses in the same locations.
Also be aware that the rules here are not at all the same as in the United States. The issues you run into whether they benefit you, or go against you are not the same issues you might run into in Canada or Europe, etc.
I dont think there is any across the board “good” advice, other than, before making any major decisions, come down first and check it out in person. Stay a little while in the areas you are thinking about being in, and do a LOT of talking to locals, or other expats in the area who may have similar businesses or who have done similar things.