Last week in September should be good on all Skeena tributaries as long as the weather cooperates. If you have a truck in Terrace, the Copper offers over 30km of road accessible water...will be people around, but you should be able to find good walk-in water. The Copper is floatable in certain stretches, but maybe not the best idea with your 80 year old father onboard. The guys at Fish Tales in Terrace can set you up with some ideas around there as well.
Bulkley will have fish in it from top to bottom during the last week of September. Common access points are pretty well known and have boat ramps...Walcott, Quick, Telkwa, Smithers Bridge, etc. Your problem floating will be shuttles, since nobody offers them (considered illegal guiding under a very strict interpretation of the regulation). Some folks have success using taxis if you're in the Smithers/Telkwa area. Oscars Source for Sports or McBike & Sport are the guys to talk to in Smithers if you want some general intel on the area.
Kispiox offers a good amount of walk-in water, but can be a touch trickier to wade than the Bulkley (a possible concern with your father). If you want to get away from the crowds for a day, you can pay the Kispiox tribe $100CAD per head (if you're a non-res alien) to access the lower 4 miles or so of the river. Offers walk-in to some of the household name runs...Upper/Lower Potato Patch, Date Creek, Powerline, Gold Room, etc. Talk to Sheldon or Monty (Kispiox Tribe River Stewards) and they can tell you how to get into the runs. Be forewarned, driving into some of those spots is borderline and the roads seem to get worse each year...again, ask the guys about the road condition before going in. If you don't have 4WD and good tires you can forget about driving in to a few of those spots.
If you have some time steelheading under your belt the gig will be pretty straightforward.
