A few weeks ago, I told my kids about a story I heard on the radio about a man who found a 100 year-old message in a bottle on the Baltic Sea. As I told them the details, I could almost hear their little minds churning and one of them started rummaging through the recycling. The next hour involved note writing, foraging through trinkets and treasures and convincing my middle daughter that a Hot Wheels truck could not fit through the neck of a wine bottle. So when I wasn't looking, she opted for a 5 dollar bill instead. We have the benefit of living a little less than 2 miles from the Pacific, so their bottle hit the water later in the afternoon.
Share this idea with your kids. I think this could be fun to do with all ages. If you don't live near water, bury a time capsule instead. To preserve creativity, don't share what items you want to float/bury until it's time to load up the container. Even if somebody wants to send something off that won't fit, seeing what they would want to share with someone else (and why) can be really eye-opening, if not endearing.
Have a great week!