One day, lots of people, food, prizes, etc. Keep everyone there as long as possible. Do the actual ribbon cutting at hour 2 of the 3 hour event. We had a well known local cafe/catering company do the food (burgers and dogs) so we didn't have to worry about that. Had a keg and drinks.
My best/worst idea for a demonstration was to wait until hour one when we had a good number of people, took a 12 foot ladder to the edge of the crown, snapped a picture with my camera phone, emailed it to the design
computer, put together a banner, printed it, finished it, and showed it to the crowd 20 minutes later. People were amazed how fast it was possible to pull that off and loved finding themselves in the picture. I put the date on it and a "thanks for coming" message and had it on the fence as everyone left.