Do you have access to a powerpoint during your seminar?
When I have done seminars in the past, I bring my fan boat to the show, a bunch of bowfishing bows (setup with spincast, retriever reels, and hand wrap reels). Have arrows sitting there...explain how the points work. Somehow you have to explain to them the aim low factor, this is where a powerpoint presentation can help you.
I also will bring my laptop and play bowfishing videos (I try to contact the manufacturer of the DVD to make sure they are alright with me playing it).
And showcase lots of pictures. Big fish is usually a good draw to get peoples attention. And fish mounts. Talk about how fun it is for youth bowfisherman, and get some handouts for information on local and national bowfishing clubs to hang out.
Some seminars people do the water tank idea, lets people shoot bowfishing bows, etc. Options are endless really.
X2 on the powerpoint slideshow. I use them for my bowfishing seminars including pictures of what I am talking about. As far as being able to let people try out what you are talking about, depending on where you hold your seminar, it could be difficult. Most of my seminars are held at sporting goods retailers (Bass Pro Shops, Sportsmans Warehouse, and the like) which makes it tough to have a place to let people try it out. I have been asked by one of my local State Parks to have a seminar at the park for people interested in getting into bowfishing. I plan on having several boats available at the lake with other members of Colorado Bowfishing Association to take people out after the seminar and give bowfishing a try.
Bring your boat - if space allows. Bring as much equipment as possible. One note on equipment - be CAREFUL that people don't pick it up and hurt themselves or others around them if you're talking to someone else. We had a 2x4 with drilled holes that allowed for smaller arrow shafts with different tips to sit in.
We've done powerpoints and that works really well. Like Brady said - people are drawn to big fish pictures. Make sure the pictures are cleaned up and not all bloody. Bloody pics may turn some people off to the sport. When we did our powerpoint we had saltwater fish and freshwater, along with bullfrogs, snapping turtles, etc pictures. We also did a basics of boats, raised/flush decks, bows, reels, rests, arrows, points, etc. Make sure that your know your states laws as to what is legal to shoot in your state. Brochures or websites to any local clubs in your area is good to have as well.
If you want people to actually "try it" - if there's a body of water near by, put out some pop bottles/milk jugs and let them shoot them from the shore, dock, etc. I think I've seen pictures that people had a stock tank that kids could shoot at a target in. ???