I would finish up on Maui. For me it is the best of the islands.
That entirely depends, it seems to me, on what one means by "best." Maui for me has been far too California-ized in too many places. The joke I used to hear in the Bay Area was that if you wanted to see your neighbor, go to Maui. If you want "old Hawaii" (and not simply the resort tourism version of "the best") then Moloka`i or the Big Island (east side) would probably be best.
Driving the north shore of Oahu (taking the H3 there, then back along the coast) past the Ranch & then the Cultural Center, the green mountains resemble those of Kauai (mini Napali!).
I'd definitely recommend a stay in a cottage in Volcano (not in the VNP itself) then a drive through Hilo and a drive on Saddle Rd. Just beware you may have a maniac in a 4WD behind you, wanting to go 70MPH on the twisty bits. A little too fast for most people, IMO!
We were very happy with Hawaii-Kauai-Oahu (would spend a little more in Hawaii, less in Kauai and more in Oahu next time, maybe equal for each).
Surprisingly, I really liked the Keahou-Captain Cook area south of KOA best on Hawaii and the North Shore of Oahu, for the scenery and the old-style atmosphere. YMMV.
How long were you on Oahu? You thought a week on Kauai was too long? (we're going for a week but that's including arriving at 9:30PM the earliest from the East coast so that in essence will be a "killed day"). Since it will be our second visit (first one was 5 nights where the last day was spent mainly at Wilcox ER... wouldn't count that as a day and arriving late from Hilo) I'm not sure if we'll feel it's "too long". Planning on major "chilling" this time around though.
How was the drive on Saddle Road? We went the northern route since we were staying at HWV and wanted to stop at Akaka Falls (had outbound flight from ITO the next afternoon after staying one night at VV) and we were "warned". Guess next time we'll try it (which will probably be our 3rd trip to Hawaii.. and will probably finally get to Maui then).
Any suggestions for "must does" on Kauai and Oahu? (we did a short drive to canyon last trip since Elizabeth was feeling ill that morning and we were trying to decide whether a trip to the ER was necessary <called her Ped. in CT to check since the hotel said there was NO walk in clinics in Poipu open -- Dec. 24th). Did a quick drive to Princeville and will probably drive up there again.