Here's my dinky little time capsule from 1991: (Of course, it looked much nicer when I was 7 years old. Most of the super cute stickers were torn off when I ripped away the packaging tape.)
Every year, I have students bring in coffee cans...I spray paint them...and we stuff them with various things from the year. I've decorated them in many ways throughout the years, but recently started adhering our class picture to the can. This year, I'm thinking of adding thumbprints to the cans and have each child sign their name next to their thumbprint.
I'll attach a note to the top, reminding the students not to open their time capsules until they graduate from high school. Then, they will open their can and reminisce about their last year of elementary school.