For some really long term storage, maybe inspiration: Introduction - 10,000 Year Clock - The Long Now
Hiding inside a museum artifact can create all sorts of plot fun. Dropped into an ocean bed with a pre-set activation of a radio beacon.
Grave / urn option seems best though as mentioned. Most places you can get a garantuee people will leave your plot alone for 40+ years.
Or simply give it to a relative / friend. Or in the walls of a newly constructed building (hope it doesn't get blown up by terrorists).
Container should be plastic or ceramic.