So this isn't The Most Amazing Advent Calendar Ever Made, but I am still pretty happy with how things turned out. They're for my 4&6 year old cousins.
I had a bunch of empty mint tins around - left over from wedding favors I never made - so first I filled them with tons of dollar store stuff. Second I wrapped two magnetic marker boards with wrapping paper. Last I put paper and numbers on the tops of the tins and magnets on the bottom. That's it!