Aloha!
You must be getting pretty excited! I've worked on both islands and there are some great diving and snorkeling opportunities on both for sure. One adventure that I always suggest to people is the WHOA charter (Wild Hawaiian Ocean Adventures). It's a snorkel adventure like no other! The staff is fantastic especially Deron. They take you to hidden gems where there are opportunities for in water time with critter like Mantas, Grey Reef Sharks, Rough Toothed Dolphins, Pilot Whales, False Killers etc... As it's the ocean you never know what you may see but nobody has the frequency and success rate of these guys. The boat is fast and super comfortable. The Humpback Whales will still be around as well which is an added bonus. The two of you could spend time in the water together this way for a memorable experience. I'm a long time dive instructor in the islands and it's the only non-diving activity that I do over there.
As far as Kauai (my home) ...
You could both enjoy the turtle habitat of Koloa Landing where you are sure to have up close and personal time with the giant Green Sea Turtles there. It's a protected bay that is loaded with all kind of amazing critters. Often we'll get a pair of visitors where one is a non-diver and they can both enjoy the water there above and below. It's located on the south shore near Poipu/Lawai. Kauai's boat dives CAN be more adventurous as the surface isn't nearly as calm on the regular basis that you find on Big Isle (Yes, we are jealous of them). Often the first dive is not snorkel friendly because of depth or being a drift. I feel like most of the Kona dives are close to shore offering a more ideal situation for a snorkeling / diving experience.
I've really enjoyed diving with Big Island Divers in the past especially with Corrine. She's really great with divers.
If I can help you with more info on Kauai please feel free to send a PM direct to me and I'd love to help you with anything I can. I've been instructing here in the islands and have a fair amount of knowledge (mostly from sea level down admittedly). It certainly doesn't have to be dive related either if you have any questions.
Have a wonderful day
A