Also the similar one with a bit more calculus, where you slice a circle (well, a disk to be precise) into an even number n of identical sectors and reorder them

Also the similar one with a bit more calculus, where you slice a circle (well, a disk to be precise) into an even number n of identical sectors and reorder them to form a parallelogram-ish shape (with "bumpy" bases) that approaches a rectangle with side lengths r and πr as n → ∞. This is an easy way to visualize why the enclosed area of a circle is πr². (You do have to take as a given that the circumference equals 2πr though.)