If you don't love the ocean (diving, fishing, boogieboarding and surfing, swimming, kayaking, nature studies - fish and tidepools and reef, etc., water and jetskiing), there is always Mt. Haleakala 12,000 above sea level (high enough for some snow - you can snowboard the Big Island in Winter) so it's a quick Mt Bike ride to the beach (depending on your fitness). And skydiving. And hiking.
Some people get "island fever." The islands are small. If you like the big city scene you will not find it like NYC or LA. Certainly NOT on Maui and only a taste of it on crowded Oahu, Honolulu, 800,000 people, with traffic jams. Still there is a bit of a night life and some very talented people in the entertainment field.
Because I am a surfer I did not miss the Mainland. I could always find waves somewhere on Oahu within 1 1/2 hours drive at the Island's other end. Maui has less and smaller waves generally than Oahu. The Kauai North Shore is more rugged than Oahu's and more remote - I got to swim the Napali Coast for 1 1/2 hours to get to surf a break by ourselves. And hike back without shoes.