Spending five days on the Big Island in October. Would appreciate hearing from Fodor's travelers as to resorts and activities for middle age individuals.

We really enjoyed a snorkel tour to Kealakekua Bay (our boat got there before the Trade Wind and others, evidently by agreement, but I don't remember the name!); it then proceeded to Place of Refuge, where the water visibility was incredible. The price was much better than the Fair Wind or Cap. Zodiak, too. They provided equipment, soft drinks, fresh fruit, and other snacks.
Up to Hawi Town and lunch at Bamboo after going to the end of the road and the valley/ocean overlook is fun.
If you go to Hilo side, there are many waterfalls, etc. right outside (upmountain, really in town).
Another favorite is to drive to Volcano National Park and check out the crater and Thurston Lava Tube( the surrounding rain forest is beautiful).
Snorkeling the volcanically heated pools (Kapoho)on the southern part of the island is also lots of fun, and a stroll through Lava Tree State Park is lovely.
Check out the Kona CC for senior rates, if you want golf.

Our absolute favorite things were seeing Akaka Falls - over 400 feet tall and then touring Volcanoes National Park. Hapuna Beach was beautiful too. We stayed at the Hapuna Beach Prince for beach time and the Kilauea Lodge for volcano time.

Many thanks for your wonderful recommendations. Looking forward to trying them. Any recommendations for a moderately price resort in the Kona area?

We have stayed north of Kona, in the Waikoloa Resort area (Fairway Villas are very nice). There are several complexes of condos (and the Marriott and Hilton) there, and some great deals on condos in particular right now. (Kona just seemed far too crowded and the traffic too busy for us.)
BTW, the name of the boat tour (the one that gets to be first in Kealakedua Bay) we enjoyed was "Dolphin Discovery"; although that wasn't the focus of our tour, we did see spinners.

I don't know what "moderate" means to you in terms of price.

October is generally a shoulder season in Hawaii, so you may get some good deals.

Check on Expedia.com or kayak.com to get an idea of what pricing is out there.