I have been to Kuai, Big Island and Oahu. Have not yet been to Maui, every time I look there prices seem to be just about double any other island.
Kuai was my favorite island, I would not go back to Oahu, watching a surf competition on the North shore was pretty cool though.
There isn't a tonne of things to do on the islands, you can find a few nice hikes and a few nice waterfalls, snorkeling etc, but 2 weeks on one island, unless you are comfortable spending 4 to 5 days just laying on the beach.
For two weeks I would recommend island hopping for sure.
Highlights-
Kuai
https://www.kauai.com/kauai-tours/ka...bing-adventure and Waimea canyon hikes.
Ohau - Surf Competition and Diamond head sunrise hike.
Big island (Kona) -
https://www.viator.com/tours/Big-Isl...el/d669-8571P1 don't remember what company we use, we had booked the trip through a travel agent so we had the choice of a tour to be included, we picked this manta ray one. Honestly not something I would have just said hey lets go do this. but out of the choices it seemed the best. It was absolutely incredible! I highly recommend doing it. They have 5-8 foot wing span and come right up to the light board you are holding onto. It really is insane.