first thing that came to mind ........
Watch for the Population explosion, now that he's POPped the question!
and some others ........
POP Goes The Weasel!!
POPularity
POPeye
What about something quilty with soda POP and POPcorn fabric to fit the theme?
If you have some photos of them, copy them onto fabric and work into the design.
You could do it as a signature quilt with yours being the first,
their pertinent info (names, dates, etc.) in the centre
then all at the party could sign in!