I'm not saying that the above advice isn't bad, but I do want to point out that some kids are going to find math interesting, and some aren't. A lot of it, in my opinion, comes from their previous experiences before coming to your conference. Personally, I was the kind of kid that just 'tuned out' whenever equations started popping up on the screen, even if the instructor was entertaining. I like math when it is coupled with a broader topic - for example, I may not like sitting down and solving physics equations for no other reason than to have an answer written down. I do, however, really enjoy solving math when it produces numbers that I can pop into a CAD program and make something real.
Therefore, it would seem to me that to appeal to the widest range of audiences, your workshop should have (like you said) a strong hands-on component that is clearly outlined at the beginning. That way, students who might be discouraged by any math topics may be able to look forward to its application.
Something like a MakerBot would be hugely engaging for students. Teach them about how it works and how to make a really simple 3D object like a cube or a sphere (Technology), then have them each try to change something about it to see what it does, like making it hollow, change the thickness of the walls, break it into pieces, etc (Engineering). If the groups (and models) are small enough, maybe you can print each of their models out (make them small enough that you can print them all at once on a platform). Then they can learn about the effects of their alterations, like structural integrity and bounciness and so on.
How about each student gets a high-brightness LED, and their little 3D printed shape is a diffuser that they can glue on and wear around for the rest of the conference? Buy a few of these kits and help the kids glue their hollow shape onto the LED: https://www.adafruit.com/products/496
Anyway, there are thousands of things you can do, hopefully you can work out something easy and fun for everyone. Let us know how it goes!