If we were going I'd make my DH wear slacks or his good jeans and a shirt (long or short sleeved depending on the weather) or maybe a nice polo top but probably something with a collar.

For me black pants and a nice top, skirt and top, dress. Even nice jeans and a pretty top

Thats just me, we aren't normally very dressy people though if you're not sure give the jeans a miss and anything else should be acceptable.