REI Flash sleeping bag.
For summer, this bag is ideal. It's rated to freezing and uses 800-fill down on top, but synthetic insulation between you and your pad. That down helps keep the bag's weight below two pounds and reduces its stuffed size. It doesn't skimp on comfort, though: A large toe box provides plenty of room to move, and a full-length zip attaches to other REI bags, in case you have a romantic trail excursion in mind. [$260; rei.com]