Kauai

If you don't have back problems, the Zodiac boat tour of the Na Pali coast is incredible. I don't have a company to recommend.

I highly recommend Holoholo Charters.

Kauai Revealed is a great guidebook that covers a lot of your questions.

How long will you be there? We were there last November and split our time between Poipu and Hanalei. The west is the driest but has limited lodging, the south (Poipu area) tends to have good weather and a good concentration of hotels/restaurants, the east (Lihue) is the most populated and is closest to the airport, and the north is the rainiest but has the most tropical paradise feel (because of all that rain). In the fall/winter, the water in the north tends to be rougher compared to the south.

The Na Pali coast is the classic boat trip - but can be rough water. I believe you can do it from the south or north coasts. There are definitely surf schools in the Poipu area (but I haven't used them so can't comment on quality).

There are tons of hikes in the Waimea Canyon State Park that can work for all abilities and trailheads are easily accessible by car. On the north side, the "classic" hike is the Kalaulau trail, but that can be pretty treacherous depending on the weather and you need to make reservations ahead of time.

Longtime Kauai resident here.
The most pleasant way to do the trip along the Na Pali coast is to book with one of the companies that operate the large 56' catamarans out of Port Allen ... located near the westside town of Eleele. I used to own a small Zodiac myself back in the day and I was seldom able to get up the coast as far as Na Pali due to the prevailing winds and sea states which made any trip in a small craft a true "Victory at Sea" ordeal.
Your best chance for sunny skies and pleasant beach weather will be Poipu - the south and west shores of the island sit in the rain shadow of Mt. Wai'alale so it's typically pleasant there - even when it's raining on the north and east sides Poipu will usually be sunny. That said, some of the prettiest scenery on Kauai is the area up around Hanalei, though there's a reason why the countryside looks so green and lush... it's particularly well watered. It is glorious on a sunny day however. Splitting time between the two sides of the island would be a good choice if you have the time and the interest. I wouldn't try to do a day trip to the north shore from Poipu - traffic can be murder getting thru the Kapaa area.
I believe there are a few surf schools that operate out of the Poipu area. A google search will no doubt turn them up. FWIW, surf conditions along the south shore in October are usually pretty benign and would be ideal conditions for a beginner.
For pretty hikes head up Waimea Canyon to Kokee State Park - lots of terrific trails to choose from. It'll be a lot cooler up there too.

We did Captain Andy ‘s boat tour for the Napali coadt. Highly recommended.

Kauai Coffee was interesting. Waimea Canyon is excellent. Drive up one road and back the other. Secret Falls kayak is fun. Check to see if the lical coupon book has half price coupons.

We enjoyed the Fern Grotto boat ride even with the damage to the ferns from the hurricane ( tho it was several years ago )

I suggest Lydgate Farms. It's a fantastic Chocolate tour as well as sampling and learning about locally grown foods. It makes the sunshine markets alot more fun. Reserve ahead of time, the tours sell out. They have a store front in Kapaa and you may be able to reserve a tour there with a discount. Their chocolate is amazing. And I am not a big chocolate eater.

We prefer the Poipu area for lodging. We stayed at the Sheraton and it was an easy walk to the shops at Kukui'ula. Near this mall is the best place for poke at a dive grocery store. It is located next to an actual dive store. We've stayed at the Marriott Vacation Club which is nice, too. Sandwiched between these 2 properties is a condo property where it's a hybrid--owners rent out their places. I'm sorry I don't have the name, but google the area and it will show up as a bunch of white buildings.

Also in the Poipu area - go to the Hyatt or Hilton, use beach access parking. Walk along the heritage trail that takes you along the coast. Stunning. Also enjoy watching the Sea Turtles play in the surf.

If you ever wanted to do a helicopter tour, Kauai is the island to do it.

Seeing a whale breaching off the Na Pali coast is a real treat from the air.

Our experience with "Wings Over Kauai" was terrific; highly recommend! https://www.wingsoverkauai.com/

Conversly, do not stay at Poipu Plantation Vacation Rentals unless you are willing to have some of your possessions go missing after housekeeping has made their rounds. https://poipubeach.com/ We enjoyed that area of Kauai, but would look at other accommodations.

We used to advise visiting guests that a helicopter tour along Na Pali was the single "don't miss" activity on the island, and I'd still recommend that over a boat trip up the coast ... though both are terrific.
The helo route goes up Waimea Canyon too, so you'd have a chance to preview the hikes in Kokee park from the air.

We loved Kauai so much that we went there for two vacations---we hardly ever repeat places! We liked to stay near Princeville. On our first trip we stayed half the time in Princeville and half at Poipu Beach so we could visit Waimea Canyon. We enjoyed our day at the canyon but the Poipu area was disappointing after staying in the north. We did the Kalalau trail and thought it wasn't too hard to get to the beach. We continued to the waterfall and that seemed a lot harder, we were wiped out when we got back! But it was great. There are many wonderful hikes of all levels, you can rent a canoe or kayak, we rented bikes and rode along a beautiful ocean path.