Late to the party, sorry if you've already left. We did the Big Island last year and had some great freedives around the southeast corner of the island. The Eagle Creek XL duffel is perfect for fins. My Omer Competitions fit perfectly along the bottom, then just packed in the rest of our dive gear and some clothes with them. http://www.amazon.com/Eagle-Creek-Trave ... eek+duffel
Beyond that we mostly enjoyed
Black Sand Beach
Place of Refuge
Kona Brewery (Good food to go along with Brews so your youngsters don't get totally left in the cold)
Mornings at the coffee plantations (we were staying right next to the coffee belt in Kona, was perfect to start your day at a new farm every day)
There's a BBQ joint right on the main road just east of Kona that is really really good. We found it on the last day and wished we'd found it sooner!
Oh and wifey really enjoyed the Royal Palace tour in Kona
Have a blast & congrats on 20-years.