It should be both air tight and waterproof, sturdy and able to withstand the elements. Even a small vault would do. Next you have to figure out where you are going to put it. Many people choose to bury them in their yard, others manage to hide them in walls, closets or even heirloom furniture. But what to put in it.
Time capsules are a testimony to a place and time as told through personal effects.
I am reminded of a scene in The Stone Diaries by Carol Shields. He wrote to his three grandchildren in Ottawa for contributions. Something small, he said, and representative of the times. Little Joan, encouraged by her father, sent a two-penny postage stamp with the king on it. Warren sent a pressed maple leaf.