We really love the Grand Hyatt on Kauai. We've stayed at other places on Kauai but the Grand Hyatt is our favorite. I think their pool is fabulous - not a water park at all - just a really amazing pool with grottos and waterfalls and winding river-like features. The Kauai Sheraton Resort has just undergone a renovation. The oceanfront rooms are very nice. It's a better place now that it was before but I still like the Grand Hyatt better. The St. Regis is on the north side - great views of Bali Hai but the rooms do not have balconies. Not a fan.

On the Big Island I would highly recommend the Mauna Lani. The Fairmont Orchid is next door to it - both are on beaches. Sandy beaches are not the norm on the Big Island. The ML is old Hawaii. Very relaxing and wonderful.

I notice that some people are recommending that you visit only one island. If you have the time and don't mind giving up some of your vacation to inter-island travel I agree with going to both of these islands. Good choices!!