These are great questions! From what I’ve seen my first semester with the St. Louis cafes, sometimes the presenter has a great activity, sometimes they have some ideas that need to be fleshed out, and sometimes they have no ideas. In the last case, I’d have a discussion with them to try and tease out some potential ideas, and we do try it out on a trial run with YLT. (For reference, I’m a site coordinator and I do lots of hands-on outreach with astronomy in my other roles.)
Activities seem to go well in the middle of the presentation, but when I was a speaker before I worked with TSC, I think mine was mostly at the end. Wherever it flows naturally in the presentation seems to be best!