Circle K

Conveniently located on the way out of Kissimmee, so that you can fill up as you are headed home (or to the theme parks, I guess), and then just pull right back out and get on your way. Also a nice convenience store to boot.
They also have a sign that says they buy unused theme park passes, but we didn’t check it out, so I’m not sure how that works — with most parks having multi-​day passes that require you to give a photo id, fingerprint, retinal scan or give blood, it might be that this only works for some sort of old ticket-​based system. Still, may be worth checking into if you want to see if you can recoup a few bucks (there are also plenty of other businesses along the street that advertise the same thing).