This is a great project for your children, no matter their age. Or you could use it for yourself.
Print out one postcard for each person at your table and fill it in. Then give them to everyone before you begin your meal. Or you could take turns reading them out loud.
You could even give them to friends and family who cannot be with you this year. Or those people who provide the services you depend on. Or, well, anyone.
Especially with everything that has happened this year, this is a great time to let those you are thankful for know how much they mean to you.
