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 s 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.
The ArroyoI also like s Arroyo ($249), also rated to 30 degrees, very light at 23 ounces, excellent down fill.
To go REALLY light, check out s 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. s 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 s Lamina 35 ($140). Rated to 35, synthetic fill, two pounds, two ounces. So not bad, weight-wise, but still more than down.