You can easily whip heavy cream and salt into homemade butter using a whisk, but if you don't have one handy, you aren't totally out of luck. All you need is heavy whipping cream and a sealed jar.
Shake the jar vigorously in two-minute intervals, with two-minute breaks in between. After a few sessions, most of the cream should congeal into delicious butter. Just strain the excess liquid and enjoy. This definitely isn't the easiest way to make homemade butter, but it will give you a great workout, and it hardly requires any tools or cleaning up.