I went to my old high school last week to do some outreach for Let's Talk Science. The teacher we're partnered with requested a grade ten physics activity on optics that could be taught to grade tens in both the academic and applied streams. We had to create our own activity based on ideas we found online. It was my two volunteer partners' first time doing anything like this, and they did a great job. We all learned a few valuable lessons, though, and these are listed below for your benefit.
The first thing we did that we will never do again is combine multiple classes of different levels. The teachers wanted to give the Let's Talk Science experience to all the grade tens in the school without making us volunteers do a million hours of work, so they combined five classes into three periods. The most difficult period was one that included one academic and one applied class. The students in these two groups have such opposite learning styles that it made having a larger number of students even more problematic. If you are faced with a similar request, at least ask for the same level when combining classes if you can't avoid combining them altogether.
My activity worked out perfectly for our round-robin style of activities (each volunteer had a station and fifteen minutes with a group of up to 15 kids). Some students finished much earlier than others, but luckily, I was able to get them discussing some more meta-issues about the experiment they conducted ("What do you think might have helped us perform a better experiment?").
The other girls, however, quickly realized that their activities ended a little too quickly. By the third period, they figured out how to change them. For example, one activity involved looking through a Fresnel lens and finding a virtual and real image by moving closer and further from the lens. The leader of this activity had, at first, simply explained what the students were going to see and gave them a chance to try it. Later, she realized that she could have them figure out that the image changed themselves, and add on supplementary activities, like finding the focal length using the size and orientation of the image.
The lesson here is to make the activity exploratory rather than explanatory, and to always assume it will take less time than expected and have extra content prepared.
Finally, a very important lesson is to never underestimate the abilities of anyone. For example, the noisiest class clown in the applied class ended up asking the most interesting questions during and after the round-robin activities; he simply had a different way of learning than the other kids.
Keep these things in mind the next time you have to design your own science outreach along with the golden rules of being hands-on, interactive, and personal, and you should do well! If you have any other useful tips, leave them in the comments.