It seems like a fairly modern concept, a time capsule. I know, for example, Egypt buried their dead with belongings and objects for use in the afterlife, but that's not quite the same thing. Did any Ancient civilizations bury objects explicitly to be preserved through time for future civilizations to discover?
Are you referring only to items that were meant to be hidden and then later rediscovered after some length of time?
You could argue that many monuments in general were meant to be preserved through time for future generations, but I'm not sure that would exactly answer your question since such items were meant to be prominent. The first monument I thought of (because I recently read about it) was The Code of Hammurabi, which explicitly addresses future rulers as in this translated extract from the above link:
In future time, through all coming generations, let the king, who may be in the land, observe the words of righteousness which I have written on my monument; let him not alter the law of the land which I have given, the edicts which I have enacted; my monument let him not mar. If such a ruler have wisdom, and be able to keep his land in order, he shall observe the words which I have written in this inscription; the rule, statute, and law of the land which I have given; the decisions which I have made will this inscription show him; let him rule his subjects accordingly, speak justice to them, give right decisions, root out the miscreants and criminals from this land, and grant prosperity to his subjects.
If I can jump in with a tangential question - When elites in Ancient cultures built elaborate tombs to "provide for them in the afterlife" was their sense of eternal life in a sense also placing their stamp on the eternity of human memory?