First of all -- SO jealous you are going to Hawaii! It's one of my favorite places on earth. I went a few years ago, and even though I was traveling on business, alone, I made time to sight-see. Enjoyed every second of it.
I can totally relate to your concerns. I'm petite as far as height (5'4" in heels) but definitely robust the other way 'round

I have had no problems doing any of the things I have wanted to do.
I've parasailed and swam with dolphins. I even took a beginners' surfing lesson, in Hawaii in fact. I have never been turned away from any activity though I am sure I have been close to some of the weight limits here and there. If you are active and willing, and it sounds like you are, GO FOR IT.
Any concerns you have, just ask the tour operator. When I was in Costa Rica, my friend (tall, like you, and probably similar in weight at the time) and I were dying to zip line. We had seen conflicting information about weight limits and were worried, but the guys running the operation could not have been more helpful. They assured us that the zip lines can accommodate many hundreds more pounds than quoted; in that case, the weight limits were really guidelines to help assess whether participants can physically do the activity and safely fit in their harness. We had a BLAST.
The hard part, for me, is getting out of my own head. I hiked Diamondhead when I was on Oahu. Once I stopped wondering "Gee, is that person watching me huff and puff?" and started thinking "Who cares!" it was awesome.
Have a great time!
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