Hi, here's a few ideas you may enjoy - https://104homestead.com/new-uses-old-greeting-cards/
I found several ideas for you.
I have a post about it here https://familyfocusblog.com/5-ways-to-reuse-greeting-cards/
You could make them into Christmas ornaments, bookmarks, or tags for gifts.
There’s some interesting ideas out there, one of my favorite is to make a garland from the cards, they sometimes with charming across the mantle. If the cards are in antique or vintage and don’t want to be cut them I have seen bloggers place these cards under glass cloches or in those vintage spiky type-flower vase holders for temporary display.
I cut out the picture and use them as tags for gifts, sometimes the verse or message does a good job of making the gift more memorable. It makes gift giving more personal.
You can also decoupage the pictures onto items.
I use them the following year for gift tags. Cut around a cute image about 1/8" trim and punch a hole in the top and add a ribbon when you tie on a package. You can make postcards out of them. Cut to the proper size for postal regulations and write a note on them just as you would a postcard. (postcard stamps are cheaper than regular mail)
My sister for years has sent all unused cards and the front only of those that are used to Missionaries in the field. They make colorful teaching aids for all ages. And helps missionaries who are on a tight budget.
I have kept all the cards we have received since we have lived in this house (48 years) as an antique dealer I know that "ephemera" has a great value but it has to be kept to be valuable.
We upcycled them on an old Cutie crate and there are a gazillion uses for it. Hope this helps with some inspiration! Hugs, Holly
You could donate them to a classroom in your area if you know a teacher. I used to use them for the kids to practice writing on. Lots of times the sender only writes on one side so I would tear it off and then use the picture and clear back for the kids to practice on. They loved it and it gave them an opportunity to take them home for someone special.