What type outfit to wear to an outside wedding at 6pm?

The weather should be nice this weekend with temp. around 67-75. The wedding will be outside. My husband is actually in the wedding. The groom was a childhood friend of his, & i don't really know them well enough to ask what to wear. I was thinking about wearing a pair of knee length dressy black pants (think they are call gaucho(sp?) pants?) with a dressy pink peasant style top & black strappy sandals. Or i have a strapless summer dress that i could wear. It comes a little above my knees but i'm worried if that is too short. Any other ideas?? Thanks!!