A vase in a vase in a vase …

A stackable pen holder cup. Have a couple of them stacked, and when you need another one, just take one out. And when you don't need it any more, screw it back in.

The threads are quite steep, so they screw themselves in by gravity, but slowly due to the air cushion inside, and actually they are quite fun to fidget with!

You also can use one upside down as lid for a smaller one, but it will unscrew (by gravity) when lifted up, so that may not be the best use.

Best printed with alternating different layer heights, so they slide into each other more smoothly.

I included 10 shells, but printed only 7 myself. They look great in multicolor filament :)