How easy is it to remove car decals?

My friend is out of town for the week, and me and a few friends want to put some really cheesy decals on his car, flames and all, make it look absolutely ridiculous, but I only want to do it if they will be easily removed without damage to the paint. They will be on for like...2 days, and we have a heat gun. So is this something that can be done without damaging the car?