I wrote a VBA Macro (excel) that searches Kayak car rentals- the purpose of this is to search a large number of cities when pickup location is flexible.
Inputs are city (unlimited list- for quicker results I keep it to 100 at a time), pickup date, return date, and car type (choices are SMALL, MEDIUM, LARGE, SUV, VAN, PICKUPTRUCK, COMMERCIAL- this field can be left blank if any car type is fine).
For now it only works with the same return city. I'm hoping to find time to expand it to one way rentals as well, but if I do I'll need to limit the number of search cities to 10 or so, so that the results are returned in a readable format.
I plan to create something similar for when the pickup city is certain but the pickup/return dates are flexible.