Wife and I have been twice - in 2015 in Maui and this year on the Big Island (both times in January). Both times rented a convertible, so I second that rec from mdmh
Maui
- the the Road to Hana drive (we stayed in the northwest part of the island); the drive itself was somewhat tedious in sections (so many turns) and I probably lost a few days off my life white knuckling certain parts, but it's definitely worth it to tour the black sand beach and the bamboo forest in Haleakala National Park (Pipiwai Trail)
- Haleakala National Park summit tour and sunset watch (bring a jacket). Seems the resorts tended to prefer recommending the sunrise tours, but we couldn't bring ourselves to get up at 4 am or whatever it would have been on vacation
- from what I remember, October might not be a great time for whale watching, but February should be good and did a great snorkeling/whale watching tour and saw a lot of activity (I'm sure wherever you are staying can recommend a specific group that does them)
Big Island
- we absolutely loved the Manta Ray night tour in Kona; pretty popular activity, but watching a sunset on a boat and then snorkeling with these huge animals was really fun
- we did a dolphin experience at the Hilton in Waikoloa Village (mostly because my wife was pregnant at the time and thought the dolphins would be aware and treat her differently...they didn't); it was fine but a little short and more expensive than I thought it would be; kids would probably like it though
- our big splurge was a helicopter tour of the whole island and I'd recommend that if you've got a flexible budget