Here's an ultralight combo, poncho for cover, black contractor bag as ground cloth, and a bivy from EMK. Great summer night, except for maybe the bugs, need some deet too.
Of course, if you use those ultralight bivy's too often, they probably won't hold up. We also sometimes use a fleece bag for summer, I think it was about $15 at the Coleman outlet near us, and it's good for 45 or so with a good base layer on. Also a nice thing in the back of the car in winter.