They needed publicity shots, so rather than hire a professional photographer, they invited a couple of their friends out for beer and appetizers and asked us to bring our cameras. We spent a couple hours eatin', drinkin', shootin', and laughin'.
The whole thing was really very fun, and it just cost them the drinks and food. I'm sure they got some good shots out of it (although mine weren't chosen, so rest assured that they were able to get higher-quality shots from someone else!).
It's something to consider if you're hankering for cheap engagement photos, or even post-wedding photos in your full get-up of the two of you having fun together. A picnic would be fun...