From Maia, I learned about this concept of a "wedding tree."
Basically it's a poster with an image of a tree on it that you set up at the reception with an ink pad, a test notepad of paper, and perhaps some wipes for people to clean their hands, and they put their thumbprint along the tree branches as the "leaves" and sign their names around it. Etsy has a good number of different printable designs. I think this is a pretty neat idea, because you then end up with basically a poster that you can hang in your home.
I will not be doing this as I don't think there will be that many kids at the reception, but if you were having many younger guests and wanted to set up activities for them, this DIY coloring book using photos project would be pretty fun, I think.
And also, we got our first wedding present the other day (thank you again to the Lius, many-generational family friends on my mom's side)! Which got me to thinking about getting some nice thank you cards. Dan doesn't really get why I want non-everyday thank you cards, but I'm pushing for them anyway. I came up with a photo-based idea to be printed at Overnight Prints that's waiting on the wedding bands arriving, but I also found these letterpressed thank you cards on Etsy with that double happiness character in the background:
$3.50 per individual card, plus shipping
It's quite an elegant design, I think. I contacted the seller about making up an order of 50 cards with design but the Chinese character for thanks in the background instead of the double happiness, and she was really open to the idea, offered to customize the border colors, and also threw in a bulk order discount.