The most-climbed "route" is the south ridge, starting from the saddle between North and Middle. It follows the ridgeline until the summit pinnacles. Then you head out left (west side) and do a traverse to a gulley which shows up on your right. Cut up the gulley to the summit. This can be fairly straightforward, or spicy black ice. It is best done with a good covering of snow everywhere, as you can use pickets for protection. Otherwise, it is a sketchy affair of vertical rubble and unprotectable traversing on insecure marbles. Inexplicably, most people seem to like to climb this pile in late summer. There have been a number of accidents w/o snow cover.
Other fairly straightforward routes include the Thayer Headwall from the east (access from Pole Creek TH) and the NW Ridge. All of these are most enjoyable and safest on cold days with good snow coverage (as is the case for almost any route in the sisters). However, since you are doing the n-s traverse, you'll probably be in there in summer.
If you can find a copy of Oregon High by Jeff Thomas in the library or at Powell's, etc. it has good descriptions of these routes, with photos.