E... Hawaii is a great choice for a vacation espcially since you will be arriving when many of the holiday vacationers will be packing up and heading back to the dreaded mainland. So to answer some of your specific questions:
1) The whales tend to come to Hawaii when the most of the tourists come (in the winter) so there should be a good amount in January. I am not sure about specific operators but it shouldnt be hard to find a good one. If you are serious about whale watching may i suggest a day trip to Maui or the big island as the stretch of water that seperates those 2 islands has the largest concentration of whales. Along the Kona coast in the big island there are places where one can spot whales from shore.
2) Snorkelling is argueably my favorite activity in the islands and I, like you hate having to take a boat to the sites. Obviously Hanumma bay is the largest and most famous site, but crowds are a real problem here and I have started to avoid it, but it is definately worth a visit. Get here early before the large parking lot fills with tour busses. Since going to Hanuma bay requires either walking or taking a tram down a long steep hill I would advise you to stay away from bringing too much beach gear, just the basics to see the fish. If you like to stay away from the crowds head to Lanakai beach. Its white sand is hard to find but if you go on a weekday the largely local beachgoing population will have left the beach uncrowded. I have had average to good snorkelling experiances here.
3) Sorry, I dont golf
4) Dining.... There is a good Chinese place at the downtown Hilton, but I would hardly call it "hidden". A cheesburger in paradise is always good for a quick burger if you are downtown as well. The plumeria cafe at the Kahala mandarin oriental is a good place for a breakfast, they have exceptionally good thin pancakes in coconut syrup if you feel like a splurge.
Other things to do include a visit to Sea life park, pricey but fun on a nice day. Of course pearl harbor. I would also reccomend that you spend some time on one of the other islands. They each have a unique feel and I find that I prefer to do 2-3 per visit. I hope this is helpful and have a great trip.