Khaki or white shorts would be perfect. Or even seersucker!!! I love seersucker for the summer, it is a little dressier but SO SO SO cool and comfortable. A really cute sash belt such as:
http://www.vineyardvines.com/product...___14686_13051
would be a really cute way to accessorize and dress things up a bit. You could do a solid color one that matches your shorts to avoid clashing if you don't know the color of the shirt yet and won't find it out for a while.
A headband is also a great idea, as well as fun (but classic) bracelets and jewelry.