Book in advance for best rates and during peak periods they might not have one available if you just turn up.

Not an urban legend.

Rental car agencies reduced their fleets when the economy tanked and there are regularly shortages of rentals available during peak times such as Christmas holiday and President's Day holiday.

People who delay reserving a rental car until early December start posting about getting quotes for $1000 a week rental car rates....

This is not just Hawaii. In NYC on summer weekend they frequently run out of cars. Many city dwellers don't own one - and have to rent every weekend to go to the shore or country house. I've had friends (even with a reservation - bit for Sat afternoon) have to wait 3 hours at Hertz or Avis for cars to be trucked in from Philly.

Book early, check prices often (and rebook, if better price comes up)

My son tells me not to worry, we hear this stuff every year, and have never had a problem. So, I told him:"OK, you handle it, and I won't worry".

He just booked the car for next year,March 13 thru April 9; no problem.

Tell him to keep checking, prices drop closer to your travel dates. I have used this site before:

discounthawaiicarrental.com