Christine Hitt
Though I generally recommend that first-time visitors stick to one island, since there's more than enough to see and do, island hopping is not difficult to do. The flights -- yes, you'll be going by air to most of them -- are short, so you won't miss much of the day. And, if you start early enough, you could decide to go to an island for the day and return, rather than staying the night. It's really up to you.
On a recent trip, I spent four days on Maui, including a daytrip to Lanai, then flew to Oahu for three days. But it's easier for me to do this because my itineraries are very focused. I've traveled so many times from island to island that I don't have to see everything. I'm going, rather, for one or two specific things to do on each island. That's really the key in making your island-hopping trips a success, otherwise, trying to pack everything into a short itinerary can be exhausting.
If you can't pick one island to base your entire trip around, then pick a couple islands and work on the itinerary. If you pick Oahu and Maui, they will each need a good number of days. You could do half and half or four days on one island and three days on the other.
Lanai can be visited in a day. I'd recommend a daytrip there while you're staying on Maui. Tip: Take the earliest ferry out so you can take advantage of the full day. Book the Lanai rental car as far in advance as you can -- there aren't many.
I tend to book daytrips to Kauai, too, rather than staying the night, unless I have hiking or camping plans. When I do go for the day, I take an early flight, get breakfast at an open restaurant, then drive around to the places I want to go. On the return, I book the flight out before it gets dark, because things close early there. Once it's nighttime, the only things left to visit are restaurants.
If your plans are to go to the Big Island of Hawaii, I would treat it like Oahu and Maui and give it a few days. As its name says, it's a big island, so driving and getting around take longer. For those reasons, I wouldn't recommend it as a daytrip.
With that said, there's no right or wrong way to island-hop in Hawaii, no matter the combination of islands you choose. It just needs some smart planning and an understanding that everything might not fit into your schedule, some cuts may need to be made.