Nguna has a village feel, some nice stretches of beach, snorkel trails through exotic coral gardens and a couple of extinct volcanoes. Pele feels more remote with scattered villages, sublime beaches, a marine protected area and a Giant Clam Garden. The two islands are so close at one point that it's possible to swim between them.
Both islands are car-free and easily walkable. On Nguna, guides (1500VT) will take you up the island's extinct volcano (Mt Marou); the climb takes about three hours return and rewards with superb panoramas over the Shepherd Islands and much of Efate.
On Pele, head out in an outrigger canoe to see the villagers turtle tagging. Sponsor your own turtle, name it and see it back into the ocean.