Although the Underground Time Capsule isn't even properly water resistant (it's endcap just screws on and off without any sort of gasket), the ability to continue to add smaller items to its vault through the screw on rock left exposed above the ground is awfully cute. There's a little padlock-style date counter on the top, as well as a more or less superfluous level. It should be available for around $20. Until they release a Vault-Tec version this might be the best plastic time capsule kit on the market.
"Kids love to bury things," a company representative assured me.