We have not visited the other islands but loved the Big Island. We spent 9 nights there a few years ago (4 were for a work conference). Hapuna Beach on the Gold Coast is absolutely gorgeous and often listed as one of the top 10 beaches in the world. We stayed at the Hapuna Prince hotel (
http://www.hapunabeachhotel.com/hbp/) with the conference and would LOVE to go back - a beautiful hotel, expensive but work was paying for this. If we stayed again we would eat offsite. We had our two teens 14 and 11 with us at the time. They enjoyed the boogie boarding and the pool there and just loved the hotel in general. They also had a free van to their sister hotel where you could watch the stingrays at night in the ocean. Hapuna Beach itself is part of a state park so you can pay the entry fee and use this beach even if you aren't staying at the hotel. You do need a car on this island to get anywhere.
We also drove to other side and went to Akaka Falls State Park (free)
http://www.terragalleria.com/pacifi...-falls-state-park/akaka-falls-state-park.html a large botanical garden (
http://www.htbg.com/) (small fee) and a Macademia Nut farm which was actually really fun
http://www.maunaloa.com/visitus.htm (small fee). This side is very rainy but absolutely beautiful. The town of Hilo is also very pretty and has some very pretty gardens. We spent two days at Volcanoes National Park (National Park fee)
http://www.nps.gov/havo/ and stayed in a bed and breakfast in the town of Volcano which was a fun experience. We had our own large 3 bedroom house with beautiful gardens and went to the main house for breakfast. It was about $135/night and free breakfast (
http://www.volcanogallery.com/. The owner was a local with lots of stories we will never forget. The national park is really fascinating and we went hiking in the rain forest and drove out on Chain of Craters road at night.
We spent our last few nights in Kona at a condo which was beautiful but didn't have air conditioning so it go quite warm. At the Entertainment rate this was also only $135 and just steps away from the ocean but no beach. We went on a snorkeling trip which was worth every penny
http://www.fair-wind.com/ (used
AAA for 20% off and did the less crowded snack cruise) to a beautiful protected area where Captain Cook was killed (the snorkeling was outstanding and we saw many dolphins from the boat), shopped, did a luau (also used AAA), went to the local beach (free). We loved the diversity of this island. The Kona/Gold Coast side is all lava rock so it's not as pretty as the other islands but has it's own beauty and is sunny most of the time. The Hilo side is very rainy and lush and somewhat cool.
We didn't think groceries were that much more than any other vacation place. We did bring snacks with us in a suitcase but they didn't cost that much more there.
We missed so many things on this island as we didn't have time - the large observatory, the horse ranch, coffee plantations, etc.
Lonely Planet puts out some great books on each of the islands that might be helpful to you.
I know you wanted to spend most of your time at the beach but there is so much to do and see! Also, the sun is much more intense there - I burned in a couple of hours with sun screen (and I tan easily) and had to spend a few days away from the beach.
We have vowed to go back some day and will again visit the Big Island and hopefully, one or two others. Have a great time!