Malaita itself is large enough that most worthwhile excursions from Auki qualify as day trips or short overnight journeys rather than departures from the island. The Langa Langa Lagoon is the obvious first excursion — a morning or full day by canoe visiting the artificial island settlements to the west of Auki. The north coast, with its extensive reef systems and the Lau Lagoon (a distinct lagoonal area with its own artificial island communities), requires more time and planning but is reachable in a long day with an early start and a reliable boat.
For those wanting to venture further into the Pacific from a Malaita base, Honiara on Guadalcanal is the natural hub. It is an hour's flight or half a day by boat, and offers the WWII battlefield sites at Bloody Ridge and the Iron Bottom Sound shipwrecks that made Guadalcanal famous. The capital also has better accommodation options and onward connections to the rest of the Solomons, including the Western Province with its famous dive sites around Gizo and Marovo Lagoon.