Butter in a baby food jar is a staple hands-on activity in many history museums. Plus it gives your upper arms a work out.
This couldn't be much simpler and I know people who do this nearly daily to have fresh butter at dinner.
Keep heavy (not ultra-pasturized) cream on the counter for a day or so. At the very least, have it at room temperature. If it sits out a little, deeper flavors develop.
Fill a small jar 1/4 full or less. The cream needs headroom to crash against the sides so the fats glob together.
Shake like mad. Butter forms amazingly fast.
Shake some more. When the glob in the middle looks pale yellow and the buttermilk looks thin, pour through a sieve. Capture the buttermilk for later and place the butter in a bowl with some ice. Press with the back of a wooden spoon or butter paddle to force out the rest of the buttermilk. Salt if you like.
Ta da. Butter.
"The only thing I have to eat is Yoo-hoo and Cocoa puffs so if you want anything else, you have to bring it with you."