We were in Hawaii last summer - you'll want to go back again and again!!
Shopping - check out Hilo Hattie - they have stores on each of the islands and have all types of souvenirs as well as good prices on clothing.
While on island of Oahu, don't forget to check out the North Shore - feels like you're in the country, but only an hour away from Waikiki.
Beaches everywhere!! Would take you a year probably to visit each beach.
While on Big Island, visit Puuhonua O Honaunau National Historical Park. A sacred site which was a place of refuge for taboo violators. Interesting if you like the history of the islands.
But my biggest recommendation - visit Kauai. The Na Pali cliffs are indescribable, as well as Waimea Canyon (little grand canyon) -- the entire island is one of the most beautiful I have ever seen!
Oh yes, any waterfall you can hike to - do it! In other words, Hawaiian Islands are full of more spectacles than you can imagaine!
Barbara in Florida