Not sure what you'll be flying, but what's the routing you have in mind? Are you going to be going North/NW first towards Laramie and the Rock Springs, kinda like the Yellowstone6 Departure, then PIH, etc.
If you stop in Nampa, the fuel will be cheaper than Boise for sure, plus it's only about 20-25 mins. to downtown Boise if you want to head there. I used to work at the FBO in Nampa several years ago and we used to let people take the courtesy car overnight. Not sure if they still allow that. Send a PM to Speed Pilot, he used to work there recently.
Sun Valley would be a cool place to stop in too, but you'll definitely be paying an Arm/Leg for gas there.