Maui / Kauai
Maui
Road to Hana - leave early, eat at the fish market in Paia, watch surfers en route, JAWS (aka Pe' ahi) is off the beaten track but you may be able to find it, if you see cattle grates -> state maintained waterfall (or so I've heard), cool hikes once you get to Hana but they take a couple of hours (this is why I said to leave early), I took the south route back (one lane roads, blow horn before you go around the corner of the cliff, but not that bad and much shorter)
Sunrise at Haleakala volcano -> altitude sickness
Helicopter tour of waterfalls and island - you may want to helicopter trip on Kauai instead
Luau - Smith plantation but in Kauai
snorkling - Maui >> Kauai. Stop by Maui Dive Shop. Rent gear and use their map and ask advice. Hit 4-5 different spots. Secluded areas are sometimes the best.
Kauai
Queen's bath
sugar plantation tube / tunnel ride thru Outfitters
Yolo the river near the airport thru Outfitters
Most important: Hawaii is a great place for fun and adventure, but be careful. Lots of opportunities for mistakes. Queen's bath can be dangerous. If swimming, respect current warnings, wildlife warnings, hiking conditions, etc. Be smart. It's not publicized but many tourists are injured.
Due to injuries / lawsuits, many new laws are in effect. The blowhole in south Kauai is now "off limits" with a fine for trespassing. Ultimate Kauai Guidebook is a great source. You probably want an older edition before the fun was banned. 4th edition has the 'secret trail' to the bottom of Wailua Falls. I took it a few years ago. It's fenced now, but the locals will tell you away around if interested. I can't remember if there was a fine for crossing the fence. Lots of swings and waterfall jumping spots that no longer exist.