There aren't any organised tours that are actually based on Lanai, though there is a catamaran cruise from Lahaina on Maui, across the short channel to Lanai, with sail and dive company, Trilogy.  They are the only company with permission to bring escorted guests by boat to the beautiful Hulopo'e Marine Preserve, with one of the best beaches in the whole of the US.  Further details and the full itinerary are available on the Maui Excursions page.

Other options on Lanai are mainly self organised.  4-WD jeeps are available to hire, and Dollar have a selection of maps and suggested off-road tour routes to take, (many roads are unmade but are passable with 4WD vehicles) to visit the amazing sights, beautiful beaches and historical locations around this tranquil island.