Something really light, I should think. I know that in the Northwest I dont use a bag thats rated to below 30 degrees during the spring and fall, even when camping on snow. Currently that bag is MontBells U.L.SS. Down Hugger #3 ($279). Weighs a mere 19 ounces and packs down to the size of a large grapefruit. Fine in chilly weather, not too warm for temps in the 40s. Down fill for long life, and no, it should not matter that it gets humid.
I also like Marmots Arroyo ($249), also rated to 30 degrees, very light at 23 ounces, excellent down fill.
To go REALLY light, check out Western Mountaineerings HighLight ($270). Its a 35-degree bag that weighs 16 ounces (sick!). Half-length zipper, superlight fabric, 850-fill down. I would think it would be fine for the camping you describe. On a cool night, just bundle up with more clothes.
You go synthetic, you save $$ but start to gain weight. REIs Aura +40 ($179), for instance, is rated to 40 degrees (duh!) but still weighs nearly two pounds. So nearly twice the HighLight, with much less warmth. Or there is Mountain Hardwears Lamina 35 ($140). Rated to 35, synthetic fill, two pounds, two ounces. So not bad, weight-wise, but still more than down.
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