Aulani has some one day excursions to the other islands on their website. You may want to start your research there. Also check out Grayline to see if they have one day excursions to Kauai from Oahu and compare the two. You may see more with one than the other.
I did the Big Island Circle Tour with Volcanoes through Grayline while staying on the Big Island this summer(went to Aulani after) and some of the people on our tour had taken the plane over from Oahu. It was about a 14-15 hour day for them and a 12 hour day for us. They were dropped back off at the airport for a 7ish pm flight. Before I decided which tour company to go with, I checked Tripadvisor for reviews. You can also call the Aulani excursion desk to see which tour company they use for theirs and see if you can find it cheaper by booking it directly with the tour company.
