We have been to serveral "casual" weddings lately. Everyone wore shorts or jeans and tennis shoes or sandals. We just went to one that was held at Thresheree Park, between Edgerton and Janesville, WI. The bride wore her white dress, but, the groom and groomsment were in tan Dockers and a nice shirt. It was so much fun, you could camp and they had a train running for awhile, so you could take a train ride around the park.
Our oldest son & daughter-in-law, had their wedding reception at a park and that was "casual". We grilled brats and burgers and had chips, dips and some pasta and potatoe salad.
We are seeing more of the "casual" weddings around here. Everyone seems to like them, because they don't have to "dress up" and it feels more like a "picnic" with family and friends than a formal wedding reception.
I wouldn't "dress up". Just wear something that is comfortable and HAVE FUN!