For a birth
The minute they arrived.
Everyone else brings clothes they will grow out of by March.
Pick the minute it happened, put their name on it, sign it from yours. From $9.99, and $19.99 for the plan that adds the certificate.
Why this one works
New parents are given a great many soft objects and remember almost none of them. What they do remember, exactly, and repeat to anyone who asks, is the time on the clock.
Keeping that minute puts the number somewhere permanent, with their child's name on it and one sentence about the day. It costs less than the outfit and it is still there in twenty years.
It is also the one occasion the time capsule was built for. Write to the eighteen-year-old you have not met, seal it, and it opens by itself on that minute, that year — which is a stranger thing to be handed than any toy.