I did this - well, I wore morning dress for the ceremony, not a suit, but then I changed for the evening festivities. It feels pretty swanky to change for the evening. You could get a more fun, summery colour of suit, that will be good for many future summer weddings you'll attend as a guest. Beige, light grey, light blue, etc. If the dress is quite formal, linen/cotton is probably out, but a summer weight wool is a good one I'm sure you'll wear lots.
Also, if you're going to do a tux (even if you're going to do it during the day), sorry if I'm repeating others' advice here, read www.BlackTieGuide.com.