I just ran across this --
http://quiltwoman.com/june-wedding-quilt-pattern.aspx -- which I think is really pretty.
Are your son and fiance traditional or non in terms of style? Do you know what colors they like to decorate with (bright vs. soft, etc.)?
The first thing that leapt to my mind was a reverse double wedding ring -- the rings plain ivory or light gold fabric, the background apple print fabrics. Or you could do an assortment of squares -- alternating apples (using snowball blocks) and hearts, for example.