I did a quilt like this for my parents' 40th wedding anniversary - sent squares to family and close friends. Got back a wonderful assortment of painted, embroidered, photo squares, lots of variety. Make sure you leave enough space around the edge for the seams! I sent mine out with the edges marked so the people wouldn't put their designs where they would end up in seams. It would also be good to ensure whatever goes into the square, can be washed.
Definitely find something that is special between the wedding couple and the person who the square is from, but I'd leave the date etc to the person putting the quilt together.