Visit the Beach, the top of the Fire Island Lighthouse, and the Sunken Forest.
Fire Island is basically a sand bar. House here have names, like boats. Cars are not allowed here in the summer so its quiet. You can just walk around on the beach or the sidewalk, but watch out for the bicycles since this is the only real way to get around. Hanging out at the beach is what Fire Island is all about. You can drive to Robert Moses State park at the West end of the beach. But the main access to the Island is by ferry. I just like to walk around and see all the interesting houses and watch people hang out on the deck.
Quick Tips:
I love to smell the ocean air, watch the deer roam around, visit the Fire Island Lighthouse (Part of National Park Service), and visit the Sunken Forest (where the forest looks like it's below the ocean).
Best Way To Get Around:
Since there are no cars here, the only ways to get around are bicycles, ferries, water taxis, and just walking.