I have a "business casual" internship this summer, so I'm going for dresses with cardigans, mostly. I think for my first day, I'll wear a gray and black watercolor-like print dress (Target) with black blazer.
I bought a new dress today, bright blue with poppies and 3/4 sleeves, from T.J. Maxx. It's on the short side, so I'll wear it with leggings.
I agree that dresses are great for summer.