There are many ways to be sustainable and ways to up-cycle your greeting cards once you've used them. If you love a creative project and want to be sustainable as possible I think you're going to like whats coming up.
1. Frame your Card
It's an affordable way to have lovely Cornish art in your house and you're reusing your greeting card without anyone know its a card! Frame is from IKEA, here is the link (https://www.ikea.com/gb/en/p/ribba-frame-white-30481875/). Frame is only £2.50, so for a fiver you can start adding a bit of Cornwall to your home.
2. Paper Bunting
If it's your friends or family member's birthday coming up, instead of buying birthday decorations, why not reuse what you already have? Simply cut your card into triangles of your choice, then sewn them together to create of lovely bit of bunting.
An easy option is just to simply recycle the greeting card ready for collection from your local council, card is widely recycled so you won't have any problems with it.