I really, really wish that I had "Adventures in FileMaking #3 - Many to Many relationships" finished and ready for release.
I'd set this up to replace the check box field with a portal that looked and acted like a set of check boxes, but clicking a "check box" in this portal is clicking a button that performs a script. That script would then add or delete records in a join table to "relate" or "unrelate" an activity to/from the current event. This would be a much simpler process than looping through all the selected values in your value list and definitely less complicated than identifying which values are NOT selected in order to do something with them.
You can get a bit of an introduction to this method in the "check boxes with scroll bars" examples found in "Adventures in FileMaking #2 - Enhanced Value Selection", but these examples add/remove values from a list in a field where here, I'd create/delete records in a join table...
Thanks for the quick reply (as always). I'll check that out. I had initially tried doing something with a portal, but I didn't know how to get the as-yet-unrelated activities to show up. I tried making fields on each table called c_Activity (calc: "Activity") and filtering the portal to show those records if they were equal but it didn't work. I may not have related the tables… it still seems like that should work. I'll poke around some more. Thanks!