Laurie, I think the ideas above are far more creative and interesting, but here is what I have done before. I get the bottle of sand, sand dollars, and the post card with "The Legend Of The Sand Dollar" in about once a year.
I rumple up a paper towel soaked in water based glue, then spread it out on a mounting board like sand lays out on the beach where it is wind blown - sort of rumply. Take the sand and sprinkle it over the paper towel. The glue makes the sand stick to it, and, viola, instant beach! If there is not enough sand, I add a little of my own into the mix.
Then I glue on whatever beach finds they bring back - sticks, seaweed, shells, rocks, etc. I try to make it look like they might be if you were looking down at the beach while walking.
For the "legend" part, I use a broken sand dollar with the little "doves" spilling out to illustrate the poem, also glued to the fake beach with all the other beach finds.
Next I cut a mat with an opening to show the "beach scene" and an opening for the "Legend Of The Sand Dollar" postcard. This mat is placed over a FC spacer mat to allow enough space for the beach treasures glued to the fake sand beach.
The spacer mat gives a place for sand grains, dried seaweed bits, etc. to fall and be hidden as the picture "ages". The legend postcard can be mounted in its opening up tight to the mat opening or spaced away.
Pick a frame deep enough for FC backing, fake sand beach backing, FC spacer(s), and glass. Be sure to start the holes for your hanging hardware BEFORE you put the assembled parts into the frame. I usually stack it all up, tape the edges along the sides in the sandwich fit (see Vivian Kistlers books), and then put the frame on, everything still face up, and staple it in, face up, by letting the frame hang over the edge of the fitting table and stapling from underneath. With the holes for the hardware predrilled, you can easily put the hanging hardware on from underneath, or by letting it stand up on it's own. You never have to turn it on it's face when fitting.
If you do this, could you add in an "engagement photo" instead of the "Legend" postcard?
Hope this helps! It sounds like a fun project!