You're literally going to the mother of all trout streams. The strain of rainbows that were originally planted all over the rockies came from the Sacramento and McCloud Rivers.
Fish the Sac in the area around Dunsmuir. There used to be a flyshop in Dunsmuir owned and operated by the legendary Ted Fay. I think Ted's left us, but the shop may still be there. If not, I seem to remember a hardware store in town that sold licences and flies. They should be able to offer advice and directions if you want to check out other great rivers in the area, including the McCloud, the Pit, Hat Creek, or the Fall River (the Fall offers very little access without a boat).
If you can't find any advice in Dunsmuir, fish the Sac in town and downstream, or try to find your way to the Nature Conservancy water on the McCloud. At The Fly Shop in Redding, you can get info on the Pit or Hat Creek. Hat is a classic spring creek, flat water, prolific bugs, and lots of big, fussy fish. It's where the big boys play, and so hard to resist.
The upper Sac is where I caught my first trout on a fly. It holds a special place in my heart, so please treat it well. It will repay the favor.