It reminds me of a mix between Zelda and Final Fantasy, and in the South Pacific. You control a single character, and the view is just like in Zelda, you walk around, talk to people, go in houses, etc. As you progress in talking to people, you'll end up getting a quest, like "Go into the cave and retrieve the cure for us". You start off with a yo-yo to fight with, and when you go into a cave, the view changes to a zoomed in one, although still arial. Often you have to kill everything in the room before you can progress, you can jump over 1 block of water or lava, and some rooms have secret passages hidden within them. Once you get so far into the game, you start getting more things, such as better weaponry, and water vehicles to go from island to island in the map view. At the same time, gameplay becomes more difficult. All sorts of really interesting things happen on your journey, and there is much to figure out in order to solve the game and see the ending. Watching the ending of this game is really worth it because it's one of the most magnificent that I've seen for an NES game.
Here are some screenshots.
This is the normal map view.
This is the view you get inside caves and dungeons.
Definitely worth checking out if you've never played it before. Also, never play StarTropics 2, just as a warning. One of the worst NES games I ever played, especially compared to its predecessor.