Most of the people in attendence will know about it, but some probably won't, so they will be surprised right along with dh.
I found an ecru linen dress with a lace overlay last year, marked down from $120 to $25. It's pretty but not too dressy, as this will be a casual affair. We had the whole shebang at our wedding - formal attire, service, reception. This time around, I want us to have fun and enjoy the experience.