I have been to Cuba 4 times since 2008 and have learned quite a bit about the country. When interacting with the locals you may feel compelled to help them out by giving away items like old clothing. You can refer to my article Things to Donate when Vacationing in Cuba to get an idea of what else they might need. Many people there will be too proud to accept a pure donation and will insist that they give something to you in return, either in a direct barter arrangement or as a gift in return for your kindness. While the people there do not have many possessions, they know how to live off of the land and their gift will likely consist of something from the land.
Taking into consideration your country's limit on bringing in foreign flora or fauna, these are some gift ideas you might consider from the people or inquiring about where you can purchase these items.
1. Lobster or other high-end seafood not available at your resort. If your new Cuban friends are experienced fishermen they will likely invite you into their homes for lobster or other seafood that they catch. Take advantage of this opportunity when you can! An alternative to this might be that they invite you to a pig roast.
2. Coffee or sugar. I am not a coffee drinker so I can't specifically comment on its quality but I have heard that their coffee is quite superior. My travel buddy always takes a package of sugar along with him from Cuba as his mom uses it for baking. They insist that the sugar is superior because it is not processed like it is in North America - it is 100% natural sugar. Keep in mind that under their current government, the Cuban people are restricted in the businesses they can partake in and the cash income they generate from it. When they offer sugar or coffee as a barter for other items you give them instead of accepting cash, that may be their means of survival.
3. Mangoes. It's more of a gesture from them than as a requirement, but after giving something to someone, they may actually climb a tree and grab some mangoes for you and cut them open for you to eat. It's more of a way of saying thanks that you can accept rather than something you should actively seek out. Your resort restaurant will have no shortage of mangoes when they are in-season from May-August.
4. Shells and other sea souvenirs. Experienced swimmers will be able to get you wonderful natural souvenirs from the ocean like shells. While you can accept them, make sure you are able to bring them back through your home country's customs. To date, I have never had a problem with customs trying to bring anything back from Cuba that is from the land when coming home to Canada.
There are other things you might search for or get back as a gift in return like hand made knick-knacks and such. You may be able to get Cuban Rum from locals as a gift. If you receive cigars, don't expect them to be real. If you plan on taking them home with you, there's a chance that they could get seized at the border so do it at your own risk.
Refer to my other articles on travelling in Cuba if you are interested in information about: