You might want to read some of the trip reports for Aulani as people have been posting their excursions there.
We were at Aulani for a week in September and we just rented a car for the week and drove ourselves to Pearl Harbor (buy tickets ahead otherwise you could end up spending hours there waiting to take the boat out to the Arizona Memorial,
http://www.recreation.gov/tourParkDetail.do?contractCode=NRSO&parkId=72369
We drove to the north shore and to Pearl Harbor. The concierge at Aulani will recommend places to snorkel and beaches to check out. You can hike Diamond Head on your own. Many have mentioned that on their trip reports. And you can do a catamaran snorkel through the Ko Olina Marina.
http://www.koolinamarina.com They also offer dolphin and whale watching tours (but the whales are not in Hawaii year round). My DSIL and family took a tour through Aulani to Waimae Falls, where they hiked to the falls, swam in the pool and hiked back through a Bamboo Forest. They had a wonderful time.
At the resort in the lagoon you can rent a paddle board (size of a small surf board) and take turns using that. These were pretty popular. There are free boogie boards available and free umbrellas. Your boys will probably enjoy the lazy river at Aulani. It was really popular with teens, kids, parents and grandparents. Aulani has a small catamaran that you can rent a spot to paddle or just take a ride on it. But it's very expensive as it's always manned by Aulani employees.
Definitely read some of the TRs, they'll give you some good ideas for your family.