You are really going to have to spend money on both Kauai and The Big Island to get out and see anything. Neither has useable public transportation.

Either suck it up and rent a car, or plan on booking tours.

Which airport on the Big Island? There are cars available in Kona (not at the airport) but it's over $1000 for your timeframe.

I just looked at travelocity. Hopefully you are flying into Kona for the Big Island. Enterprise has cars for $10-20 per day plus fees if you pick up on the 28th and drop off on the 31st. Just take a cab to/from the airport.

Where are you staying and what did you want to do on the Big Island? If you just want to sit at a resort and go to the beach, no car is needed. But if you want to go explore the island you are either going to need a car or take tours everywhere.

Forget about that Enterprise price. Even though I entered Kona and it showed the Enterprise location as being 5 miles away (in Kailua), the Kailua location with that price is actually near Honolulu. Not even sure how that happened but whatever, the location near Kona is sold out.

Try www.discounthawaiicarrental.com

It's a decent agency and you might find something.

Don't wait. There will be absolutely nothing.

aloha from kauai...inventory is the problem on all islands. i recommend to my guests to grab any car months before they arrive. Christmas is just crazy here and i end up shuttling for the entire time folks unable to find a car. Taxis are super expensive, there is a local bus that is fairly helpful however limited in pick up spots. Some locals advertise on craigslist their private autos and you add n your insurance...kinda if'y but a choice nonetheless...some budget cheap places off the radar should be avoided as they have zero customer service concerns...keep looking and grab what you can! you need a car for ease of flow...a hui ho

And yes, you absolutely will need/want a car on both islands.

We will be staying at the Hilton Waikoloa. We like to go around and try to explore the island as much as possible. We are flying into Kona airport. There is a National car rental agency at the resort and when I called them they gave me a price of around $430 for 4 days. Should we just get it?

If you don't someone else will.

It will probably cost at least $65 taxi fare from the Kona airport to the Hilton Waikoloa. And the same back.

Well, rentals at Kona airport are over $1000 for those days so $430 is a bargain. Might as well book it then keep looking for other options, you could try Priceline or Hotwire.

If that car is right at your resort, you could see what the price is for 3 days - pick up on the 28th and return on the 31st. You're not going to want to drive around the island every day, might as well commit to spending the first/last day at the resort and save some money on the rental that way.