What tour is best of the Big Island, Hawaii during a 8 hr stay from cruise ship.

What port is the stop?

It is Maui ( Haleakela) that has the bike down the mountain trip.

If the stop is on the Big Island, there is no bike tour.

I think that you are mixing up the Big Island and Maui. Maui has Haleakala NP. However, after a number of fatalities the NPS stopped commercial bike tours within the National Park so you cannot ride downhill from the top. Commercial touring companies like Bike Maui offer downhill rides from the NP entrance: https://www.bikemaui.com/bike-tour-route/

Are you talking about tours out of Hilo? I did a google search and found many tours but not what your hubby wants to do. Just remember that many bike tours have age restrictions due to insurance issues. Honestly, any tour catering to the cruise passenger would have a pickup place near the port.

You are right to be concerned about being late getting back to the ship though. I did a tour in Iceland through a private company. I was the last person on the ship. Whew!

The bike ride down the Volcano, Haleakala on Maui is a rather steep ride and not all that pretty in spots.

Edited to say that the post above says they are no longer offered in the park.

As noted above the bike into a old volcano is a Maui thing. The Big Island (Hawaii) has the active volcano.

I found cruise tours to be excessively expensive and time driven. By that I mean when I wanted to take time to study something we were forced to move on because they have to do the "whole" tour itinerary. (Which is why I don't do package tours anymore.)

The problem is the volcano park is approximately half way between Kona and Hilo, the two major cities. It's over an hours drive from either, and if you add time for getting a rental car and turning it in, that leaves you with about 4 hours of time to drive, visit, and explore. Still one of the main reasons people go to Hawaii is to see the active volcano (or what you can see as it's not always actually spouting lava in an accessible area). To me, the National Park is one of the best places to visit on that island. The Visitor Center is informative, and they often have cultural events being held there (check the schedule). There are volcanic tubes to explore, a chance to walk on the newest solid ground added to the island (and thus, to the USA), and dozens of scenic viewpoints and walks.

If you dock near Hilo you'll find it is very different than the Kona side of the island. Hilo is rain forest and small town, Kona is dry, tourist resorts, and golf courses. Just walking around downtown Hilo can be a lot of fun. It has the best open air market on the island, art galleries, shops, a couple good micro breweries, a really nice park along the water, and I always see turtles when I'm in Hilo.

Two of the bigger tours are the "Ranch" in the central part of the island, which I found touristy and not very exciting (I've seen a gift shop before), and the trip up to the observatory at the peak of the mountain. If you're a major astronomy buff that's kind of interesting, but the ride is fairly boring and the air is very thin, so if you have breathing problems I wouldn't suggest it.

Public transportation is not going to be very practical. If you dock in Hilo there's a bus that goes to the Volcanoes NP, and it takes a little over an hour each way. Very cheap. But they are also on a limited schedule that may not work for you. There's a bus from Kona too, but it takes 3 hours each way due to stops, so that's not practical with your time constraints. I think rental car is the way to go. If you can get Enterprise to bring you the car that would save a lot of time.

Another suggestion would be to see about local van tours. The first time I was on the island I signed up for one and was the only person who showed, so basically I had a local guide who showed me the island and what I wanted to see for a day for about $40. Then I drove myself back to all the places I wanted to spend more time in.

Last, if either of you are ex-military you can use the MWR facilities, including tours.

As asked above what is/are the port(s) of call? The Port of Hilo is the Big Island's shipping port that I believe cruise ships can dock. Some cruise ships anchor off of Kona and boat the tourist in to shore because Kona does not have a deep water dock.

If Hilo is your port of call you will dock very close to the town of Hilo and as others note they are walkabout things to see and do at budget prices. The Hilo Farmers Market is at the top of my list for Hilo with Wednesday and Saturday being the big market days: http://hilofarmersmarket.com/

The Volcano NP is closer to Hilo than Kona and our must see/do for any Big Island excursion. Where else can you see and experience an active volcano that is relatively safe? So a Goggle search for "volcano national park tours from hilo".

A lot to see and do on the Big Island. Just don't miss the boat.

I would bite the bullet and spend the money for the tour offered by the cruise ship. If you go on your own and miss the boat, it will be expensive to fly to another island and rejoin the ship.