I prefer summer travel, too, myself
We haven't had a ton of discussions here about Hawaii since most people don't really roadtrip there. :) But here's a discussion about it that might give you a few tips.
Most people fly an island-hopper between the islands although I'm sure there are some boats you can take between them and that would be very cool. I don't have any information about that though.
With a few weeks, you'll easily have time to see all of the islands.
It's hard to choose as they all offer something different and have a different feel, somewhat different topography, etc. so each is worth a visit.
I don't believe there's all that much difference between car hires. Go for price/type of vehicle. And you'll definitely want a car at each island.