The responsibility for the hands on activity ultimately is shared among the adult leader, presenter and teens. The presenter may have an idea, but the teen leaders will be able to help refine it. If the presenter does not have ideas, the adult leader may need to dig into the topic and see if there are any suitable examples in digital libraries for STEM or other places.
Ideally, discussion of a hands on activity is part of the initial contact with the presenter. Like Nicole said, some have ideas, some have no ideas, some have an activity ready to go. The best topics are when the scientist has something they can do with their research. A cyber security expert who develops capture the flag activities for professional competitions, created something simpler for teens. A professional with the office of medical investigations brought skeletons to ID with a victim list. A scientist that studied craters on planets set up an experiment to determine variables that affect the crater size and depth.
Modelers who can allow the audience to change variables in their model to see what happens have been pretty popular.
But so have hands on when a person brings a collection of artifacts – such as minerals, or skulls, or brains, or holographic films.
Some topics are just hard to find a hands on. Some may be best explored in small group discussions or as a debate.
Because this is such an important element, the more we share information about our topics and the learning activity through the Resource section, a forum post or blog post, the more we will all begin to better identify strong and effective hands on activities.