XOXO

What about jeans/khakis/a long skirt, a button down shirt and boots?

What is this couple like?

I'm sure if they showed up in normal wedding attire they would be fine. It sounds like the bride and groom want a low key wedding, but they don't sound like the type who would care if their guests wore normal wedding outfits.

I agree with Aly Khakis/long skirt/ button up shirt/ boots

Tell them to bring a camera I'd be interested in seeing pics of the wedding I never thought of having a wedding at a art museum i think that sounds very cool!