Dynamically update checkboxes?

I’m building a small booking site, and I’d like to have activities as checkboxes. Activities are basically posts, so checkboxes would just be post titles.

I can enter them manually into each form, but the problem is there are many forms, and every time I add a new activity, I have to again manually add them one by one.

Is there a way to somehow dynamically populate checkboxes, in ANY WAY? I can live with creating activities in another way (not posts) if that gets the result I need.