Let's see.....
Just brainstorming, but I would say once a month. I think weekly might get a little lost, but once a month would have some momentum behind it! If there is a holiday, then that would make it convenient. Like a 3FC Turkey Trot or something.
There could be themes and that would be fun, but it would need to stay open enough that all folks of all abilities and fitness feel welcome.
Maybe there could be a "Try Something New" 5k. People could bike it if they usually run, or walk in a brand new location. Just something to break out of habits and spread our wings.
For Halloween, we could wear costumes and post pictures (I know, I am so creative sometimes that it is scary!

).
Just a couple of thoughts.