I never do anything for either warm-up or cool-down. If you're not dizzy when you get off the bike there's no need for cool-down. And biking is really such gentle exercise that there's no need for warm-up, either, unless you are racing. Your first hill will warm you up fine. If you want to stretch, do so midway on the ride, also fine to stretch at the end, but I'm usually so flexible by then that it's silly. I like to sprint at the end, sort of like your hill, and then finish gasping. It's good for you.