For a more traditional hooded down bag, I like Marmot's Arroyo ($259; www.marmot.com). It's rated to 30 degrees (last weekend I used one at 11,000 feet on Mount Rainier and was overly warm!) but is fine for summer use. It's also super light at one pound, 14 ounces, and packs down to the size of a cantaloupe. This has been my favorite all-purpose bag for about four years now.
On the synthetic front, Integral Designs' Andromeda Strain ($170 in regular length, standard width; www.integraldesigns.com) is one of the best light synthetic bags around. It's rated to 40 degrees but can handle cooler temps. The Primaloft fill is soft, compressible, and highly water resistant. It comes with a hood, but a full zip expedites unzipping. And, weighing only one pound, 14 ounces (it can be ordered in wider models, too), it's very light. It should be plenty warm for your intended use. Most other synthetics weigh in at over three pounds, or are rated at temps that make a bag almost superfluousyou might just as well carry a fleece blanket.
Hope that helps. Thanks for reading!