August in Minnesota? A mosquito net, of course! A casual wedding here in the summer...definitely a pair of lightweight, light-colored khakis and a button down short-sleeved shirt would be good (think Dockers,lol). If it's really casual, you maybe could even get away with a pair of dressier shorts and a pair of nice sandals (no socks!).
I don't know how formal your family is, but every wedding I've ever gone to it's not unusual to see people (especially guys) in jeans. Shorts, if it's hot. Oh and a Twins baseball hat too, of course. I would probably skip the hat.
I don't know if you have Kohls in your neck of the woods, but they're great for those kinds of clothes at a really good price. Don't forget the Deep-Woods Off! Have fun and let us know how "casual" it ended up being. I'm curious to see if it's the same type of casual that I know.
Chillaxin with Agman, man! March 2012