I totally agree with the recommendation to read the wizard pub books. They are IMHO the best guidebooks around. OP, it is hard to recommend which island is best for you as "beaches and sight seeing" is pretty vague. I've been to Oahu, Kauai, and the big island (TBI) the last few years. All are great but very different. I like TBI the best, but I don't mind driving a lot. TBI is pretty much the only place that you can safely see a live, active volcano.

Mikey, during that Oct-April time, avoid holidays, especially thanksgiving, christmas-new years, and spring breaks. I've found that the loads are usually light the first parts of Nov and Dec. Traveling Weds is also helpful.