I wound up with a DARK blue Taurus from Hertz this past April. Besides needing a power boost, it was fine for what we wanted to do. The only two things I can remember needing an SUV for was the Waipio valley and the obvervatories. Otherwise, we were fine with a sedan.
One point about the observatory road...and the reason the rental companies don't want you going there in a non-SUV: the air is so think up there at 13K feet that the cooling effect of the air on your cars brakes is greatly diminished. So when your coming down the road from the top and your brakes overheat and fail, you'll know why! The 4WD vehicles can use their gears to keep the car speed under control, which is why they are recommended.