Last-minute car rental question

[Miffy]

We're planning on being at WDW the second week in December, coming by train, using a car service to get to WL, and using Disney transportation while there. But we might want to have a car for a day to go off-site. If we don't book this in advance, is it hugely more expensive to rent a car on the fly while we're there? And what's the easiest place to rent from on-site or nearby?

Thank you, transportaion experts, for your help.