I started climbing last summer and the North Twin Sister was my first one. Great day, full of variety: bike up logging roads, exposed class 3/4 scramble on great rock, glissade down the north face, bike down to the car. Go earlier in the season so the north face is snow, rather than scree.
Also: Shuksan, Sahale, Eldorado, Temple/High Priest (Enchantments area)...