I've just received the coolest DIY thank you card. To top off a gorgeous wedding which had lots of homespun details (bunting, pots of jam, handmade stationery), the newlyweds sent quirky thank you photos taken on their Maldives honeymoon. They'd written 'Thank you, from Ben and Sophie x' into the sand and asked a passer by to take a picture of them next to the inscription. It was especially nice, since they'd asked for holiday activities as wedding gifts. A cool alternative is to send pictures of your guests at your wedding as thank you cards - it'll make a fab reminder of your day.
Fancy doing this yourself? Once you've chosen the snap/s, there are lots of websites that do the creative stuff for you. They have templates to choose from and you can add backgrounds, choose pretty fonts etc. You just have to upload the photo and add your message. Check out zazzle.co.uk, photobox.co.uk, optimalprint.co.uk, which are all easy to use and don't cost the earth.
Alternatively, if you're feeling super creative, want to save a few pounds and have a decent printer and suitable paper/card, you could even print them yourself at home and add handwritten messages. The result might not be as polished but your guests will appreciate the effort.