My Two Cents
I think the choice for islands would be based on the OP's preference for a resort vacation vs. an adventure trip. If the OP wanted to do the resort thing, the best island combo would be O'ahu and Maui.
For adventure, the best combo is Kaua'i and the Big Island. However, being their first trip to the Islands, I would suggest either squeezing in 1.5 days in Oahu, or extending the trip 1-2 days to accomodate a "hi-light" tour - giving the OP a chance to visit the Arizona Memorial, Waikiki, The Bishop Museum (recommended if time), or a photo tour of some of downtown's historic buildings and palaces.
I am a believer in first-timers hitting the hi-lights on the islands, then returning for more in depth stays to savor more activities (once you have a taste, you will want to go back for more). That can mean as little as 3 full days on some islands to get a 'taste'.
If you are visiting the Big Island and want a beginner's snorkel spot, Kahalu'u and Honaunau are superior to Haunama Bay, which I find way too crowded and damaged.
I also find that well timed inter-island flights do not spoil an entire day. If the trip could be extended to 12 days, here is how I would break it down...
Day 1 - Arrive from mainland, transfer to Lihue
Day 2 - tour Waimea Canyon, Kalalau Lookout, Wailua and Opaeka'a Falls
Day 3 - tour Kaua'i north shore, including Hanalei and Kalalau Trail
Day 4 - morning spouting horn and botanical gardens, balance is rest day, dining, shopping
Day 5 - early morning transfer to Kona, afternoon snorkeling on BI
Day 6 - northbound drive via Kamuela to see windward sights including Waipio Valley lookout, Akaka and Rainbow Falls, Hilo and then after early dinner take Saddle Rd to stop at Mauna Kea visitor's center for stargaze before returning to leeward side hotel
Day 7 - snorkel trip via Four Winds to Capt Cook/Kealakekua Bay then afternoon drive to South Point for Green Sand Beach hike
Day 8 - day trip to Volcano Nat'l Park
Day 9 - rest day, dining, shopping
Day 10 - morning transfer to O'ahu, Arizona Memorial, evening at leisure
Day 11 - round-the-island tour including North Shore, Halona Blowhole, Haunama Bay snorkel
Day 12 - optional Bishop Museum, photo tour of State Capital building, Kamehameha Statue, 'Iolani Palace, shopping, late evening flight to mainland
My sample itinerary has alot to do, might be abit tiring for some, but hits many of the hi-lights and gives the first time visitor enough of a feel for the islands to decide on which they want to return to and see more in depth.
Remember, you could spend an entire week on any one of the islands and still not see everything. That's why I like the first trips to be more of a 'survey', followed by more in-depth visits the next time.