Thanks. I missed this the last time you posted it. I’ve long used and passed on the trick of using a smooth rock to create a tie-down point, but, yeah, let’s use lighter material when we can. Ping-pong balls are also really light and quite strong (they can be smacked hard enough to reach 66 mph). Pop bottle screw-top caps are not as spherical, but pretty smooth and are free in the trash or at the recycling center. I usually bring an extra water-bottle cap in case I drop mine.
I’m trying to think of a multi-purpose option and can only come up with peanut M&Ms which are not so good in the rain.