As to the runs,
The Klamath is Dam-Controlled, so all of those runs are available all year long. Its a longer drive to get there, but in some ways its actually way more fun than the Trinity. For one thing, the Klamath is like 70 degrees in the summer-time (top-feed dam) so you don't need any cold-water gear(in the summer). Its all II and III, but plenty of fun and several well-known playspots, oh, and I forgot, there is a third II-III run there called the Savage Run, and below that is the Cade Canyon, which isn't hard, but is very beautiful with several good 20 and 30 foot diving rocks. Basically, on the Trinity, you just have Pigeon Point for class II-III, and then Burnt Ranch Gorge for class IV-V. The Klamath has a long list of II-III runs, you could spend four or five days on it and still not run it all...and it has a ton of tributaries you can also paddle. If you visit at the right time (when the water's in), you could, say, run Tree of Heaven on day 1, you could do the Lower Scott on day 2, maybe do the Shasta River Canyon on day 3, drive all the way down to Clear Creek (Siskiyou Co) on day 4, do the Cal Salmon on day 5, the nearby Ike's Run on the Klamath on day 6....which still leaves you the Happy Camp Run, the Savage Run, Cade Canyon, Indian Creek and the Hamburg run to do on another trip. Its a great canoe river.
The Upper Sac from Sims to Lamonine is a great class III run, and its just 40 mins or so north of Redding. Usually a spring-early summer run. I have done it at mid-summer levels, but its pretty much just a rock-scrape. Upper Box Canyon is very tight, but I would still call it III-IV...you are correct in that Lower Box Canyon is solid IV...and at the right levels, its class IV with one very gnarly V+...but you can get out before then.
The McCloud is kinda intense, very long rapids, the water is deathly cold (its glacier-fed via rock tunnels), the scenery is amazing. At 2 different places these massive waterfalls come straight out of a cliff wall and pour into the river. Each of these falls pretty much doubles the flow of the river. I've never seen anything quite like it. Later on you have the famous Hearst Castles to stare at, and they are cool too.
The Scott River feeds into the Klamath, and how much of it is Class II-III depends on the water level. The lower 5 miles or so is always class II-III, and you can paddle out of it and into the Klamath where there is another short class II-III run to continue on. At lower water, you can put in and run the lower 10 miles of the Scott and its all II-III. At higher levels some of those III's become solid IV's, and one Rapid, Schuler Gulch, becomes a serious class V. Portage is not TOO horrendous though.
I ran the Butler Creek run of the CAL Salmon one time at 1200 cfs...and it was an amazing canoe run. Good class III-IV, lots of technical stuff, and some breathing space between rapids. At higher water it is less technical, but also less breathing time. Its still a blast.
Clear Creek (Shasta County), Battle Creek(Shasta County), and the Shasta River Canyon are highly dependent on the water levels. Clear Creek is a class III-IV run, pretty exciting, technical. The Shasta River Canyon is busy class III, usually only doable in the spring. Done it just once. Battle Creek is Class III, and its just straight awesome. When Battle Creek is flowing, everybody's going...which reminds me, I haven't been there in a decade. Time to change that!
I've never done the Upper Eel, I'm just going off a friend I trust who says its a fun class II run. I always meant to get up there, but just never quite made it.
I'm always down to host a trip, anytime anyone wants to come up.
-Rob
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If you're willing to step it up just a