Something I started with my son was sending a homemade thank you card with a photo of him for birthday and holiday gifts. In the beginning, I’d make each one different with a photo of my son and gift from the person receiving the thank you (e.g., him laying on the blanket someone made him or unwrapping the toy they sent). Four years and another kid later, everyone gets the same photo, but they’re still a hit with the grandmas! I think I could do more with the design of these, but lately I feel like the photo says enough.
My designer takeaway: You can personalize a thank you with a image rather than text. (At times, we customize thank yous or certificates with images at work.)