There's great kayaking all over Oahu, but nowhere is it more accessible to visitors and locals alike than Lanikai Beach (Mokulua Drive). Rent a kayak on the beach or from a nearby shop, then paddle your way out to the Mokulua Islands for an afternoon picnic. While the trip is by no means advanced, you (or your travel partner) should be in somewhat good shape to kayak the distance. Bring snorkeling gear, and make a point to check out the tide pools and different fauna of the islands.
Kayak From Lanikai Beach to the Mokulua Islands