The cultural differences between Europeans and people of Central America are so big that you could devote an entire book to the subject. Below are some selected items that will help/be of interest to you during your dealings with the residents.
Behavior: Nicaragua is known for its friendly and talkative people. Despite the misery that they sometimes experience or the dire situation they face, they stay ready prepared with a smile. The people are open and curious and you should expect direct questions about your age, marital status, weight, salary, or your opinion about Nicaragua. Despite their directness, Nicaraguans tend to cumbersome and their behaviour is indirectly associated with the cultural idea of "saving one's face''. If you ask something they do not know the answer to they will make something up or circle around the question to avoid being ashamed (tener pena).
Greetings: In Nicaragua, offering one hand to both men and women in the first encounter is normal. Close friends or relatives will give each other a kiss. Male family members offer each other only one hand, often accompanied by an embrace, an abrazo. If someone appeals, even in situations in a shop or bus, start your conversation with a greeting. A simple buenos días or buenas tardes, accompanied by a smile will do wonders. Your greeting will be matched by a greeting from the counterparty. Then you can enter in to the actual conversation. If you enter a common space, such as a restaurant or waiting room, you should extend a general greeting to everyone in the room. A simple buenos días or buenas tardes will suffice. Politeness is a very important point in the interactions across Central America.
Clothing: Make sure you look clean and cared for the people of Central America is important. Even poor people are doing their best to be groomed. Since you have the money to travel all the way from Europe to Central America you are, in the eyes of local people rich. You win respect if you dress nicely. This does not mean that you should go out to the streets in your most expensive Armani outfit. Jewellery and especially expensive jewellery you had better leave at home. Do not dress too revealing, not everyone will appreciate it. Shorts are for both men and women well suited to the beaches and in some coastal towns. Swimsuits should only be worn in swimming pools and on beaches. Many local women swim with a shirt over their bathing suits (often to reduce male attention). Dress discreetly when you visit churches, out of respect for the local population and its culture. Dress neatly when you visit a church, cover shoulders and knees out of respect for the local population and their religion.
Macho behavior: The myth that characterizes men as strong and women as fragile remains strongly present in Central America. A macho male attitude and pride are often exhibited as a local man tries to get the attention of a woman. Unaccompanied women especially will receive plenty of (unwanted) attention. If you do not appreciate this, it is best simply to ignore it. You can minimize these problems by dressing conservatively. Wearing a wedding ring can also help. Avoid dark places and lonely beaches if you are alone. Obviously there are plenty of 'ordinary' men here too!
Buy: The many local markets in Managua and other cities are very interesting. Markets such as Roberto Huembes market in Managua, have a wide range of artesanía, hammocks, Spanish-language CDs, souvenirs and clothing. Mercado Oriental is also worth visiting, but it is chaotic and there are many relatively deprived people there. Masaya is known as the city of hammocks and has two markets filled with handicrafts from different regions. Typical souvenirs include handicrafts like wooden pots / sculptures, pottery vases, soap stone sculptures from San Juan de Limay, or a real-Masaya hammock.
Bargaining: In Nicaragua, haggling is a social sport. Keep in mind that bargaining must remain pleasant and fun. Maybe you are fooled once or you pay more than the locals, but prices are still considerably lower than in Europe. It is important that you arrange a price in Managua before you get in a taxi. In other cities such as León and Granada rates are reasonable and you may ask the locals what the usual prices are. It is wise to do that anyway. In the shops you can also haggle for items, but prices in supermarkets and buses are fixed.
Begging: In Nicaragua begging for some people is the only way to survive. This is true for old or disabled people to whom you of course can give a donation, especially when you see the locals do so. But there are also those that are begging because they are addicted to alcohol and/or drugs. With children it is in principle better not to give money but instead fruit or anything else to eat. If children get their money in this way they are much less motivated to go to school or work.
General: Always remember that you are a guest in a country where one simply has different manners. This is not abnormal, you're just acting differently!