You could also hire a couple of caricature artists to do funny sketches of people if you think the crowd would be into it. You could also do a creative guest book to occupy people and stimulate conversation:
1) If you do an internet search you'll be able to find 'instant' Polaroid guest books where each guest can take a Polaroid of themself and insert it into the guest book where they write their message. This can get fun if you have people with a sense of humor who could have a good time with the picture taking.
2) If you're crafty you could make your own guestbook---for example, my sister is a quilter and for her wedding she started a quilt, and then left out a bunch of unsewn squares with fabric markers for people to write on. After the wedding she sewed the squares into the quilt, completing a 'wedding quilt'!
3) Bubbles! Especially if you're in a spacious place or outdoors, give all the little kids bubbles and it will be really cute watching them have fun blowing bubbles. Not to mention it could make a really pretty atmosphere. Just be careful to confine it to a certain area since bubbles could land on food, leave soapy residue on glass, etc.
I'll try to think of more but those are some of the things that come to mind.