H and I are going on vacation and will be gone about 5 days. We need a car for 2 of those day (mid-week). Is it cheaper to pre-book or would it be okay to just rent one once we get there?
Get a reservation. It takes 5 minutes and it will save you time and potential headaches if you get there and some random convention is there and there are no cars.
Do not rent the car when you get there, it WILL be more expensive.
To get a price in the $7 range I use the name your own price function on priceline. I've had luck getting the $8-$10 rates tend to be for weekends and not mid-week.