We are going to Hawaii in October 04, and visiting 3 islands. Big Island 10 days, Maui 4 days, Kauai 5 days. Our car rental weekly rate (7 days) on the Big Island is great. However, then we have to pay a high daily for the 3 extra days. Same problem on Maui and Kauai because we are staying for less than a week. Is there any way to avoid the high daily rates? Will any car rental companies combine interisland rentals? Thanks for your help.

Try Priceline. We used them recently and paid a daily rate of $15($25 a day after all the taxes and fees). We rented a compact car for 10 days on Kauai. It worked for us.

Check Alamo's website before bidding on Priceline. Alamo was cheaper than what I could get on Priceline. I am paying $198.99 w/taxes and fees for a compact car for 8 days on Oahu.)

Sometimes it is cheaper to pay the rate for a whole week even if you only keep the car for five days (e.g. 7 days @ 20 = 140 vs 5 days @ $40 = $200).

Just make sure there are no early return penalties before you do this!!!

Call each major rental outfit and see if they will treat your individual stays as one long rental. I've read that one of them will do this but I don't remember which one it was, sorry
Let us know how it turns out.

Mellyz, there is no such thing as an "early return penalty" on short term car rentals. If you pay for seven days and only keep the car for five days then the car rental company is happy because they can get double rental for two days. Why would they penalize you for doing them a favour ?

Keep watching the major companies websites for specials, I have always had luck doing this. It takes some extra time, but just keep checking them and something will crop up.

Check with Roberts Hawaii/Roberts Overnighter. Especially if you are needing inter-island air.