The tour will take you around the island, walking and by boat. If you're brave enough to go there by yourself.. then remember to ask permission before you go anywhere as the island is all private property and even the water, bush, caves are owned by people. When you pay for the island tour, this contributes to the locals.
We got a small power boat across to Lelepa Island from Efate and back (without lights in the dark...hehe). The trip wasn't too long although the boat was very slow...it just gave us more time to look around and chat though. :)