How about those disposable cameras that people use at weddings? that would be awesome for everyone to take photos and then have a cd made of all of them. Possibly even add a little more $$ to the fee to pay for them.
The big thing for me would to be sure each participant had plenty of space to work and that the food was separate from this work area. It makes me nervous to have people eating where I am working.
Maybe a masseuse would be willing to come in and do mini massages in the neck and shoulder area. She could charge a nominal fee but would probably have plenty of takers. (Each participant would pay for their massage as an extra, not included in the retreat fee.)