keeps the people at bay
As you can probably tell just by looking at him, Riku is in very good physical shape. He tones his muscles by lifting around logs and other heavy objects, as Sora and Kairi are the resident bums of the island and are therefore far too lazy to help out. Riku also spars with Sora, and we can assume that he probably spars with the other trio (Wakka, Tidus, and Selphie) as well. He goes for the occasional run down the beach, with his friends sometimes joining in. According to just about everyone there, Riku is the toughest guy, and he's got a pretty long winning streak with those fights. Alas, the effort of hard work does pay off. ...Or maybe it's just his raw natural talent.
Now we know how he gets his exercise, but one knows that health also comprises of nutrition. Due to the fact that he lives on an island surrounded by water, we can assume he has fish in his diet. Fish are very healthy for you, and supposedly they increase your brainpower.
Either Sora doesn't eat fish or his head is so dense they've got no effect on him. Along with that I'd say he eats a plentiful amount of fruits from all those trees. With a healthy diet and lots of exercise, it's no wonder that he's in such great shape.