Renting a car - will making a reservation ahead of time save me much if any money?

[Carrie S]

My inlaws purchased of some tickets to visit them in FLorida. Very nice but on the way home theres a 3hr layover for a 1hr flight =4hrs (this is after already flying 4hrs) with young kids and pregnant not fun & to drive its less then 2.5 hrs. So if I cant get an earlier flight when we get there Im thinking maybe well just rent a car so we can keep heading home. I dontw ant to make a reservation because I imagine if we get an earlier flight then it maybe costly to cancel the car? However I see its already $100 if I reserve it, I wonder if it costs much mroe if I get a car that sameday? Just want to know what options Ill be looking at if any. Thanks.