what are you looking to incorporate?
ideas:
Photos from the first wedding and the second
a banner that says happily ever after (you can adhere ribbon to paper and print on a printer)
your pins
flowers like the ones she is using or used. Prima has some really beautiful vintage flowers. I saw those in the scrapbooks section at Michael's
some wedding rings like you would find at a crafts store
a copy of their invitation
gluing stuff down in a shadow box is tough. I used glue dots (you can find these in the scrapbook section)
They seemed to hold up the best.
when doing your shadow box use multiple depths and layers.