Forestry Trunk (940) south of Highwood Junction is decent gravel, but not near as scenic as the part north of there. I did that several years ago in the fall. Near Coleman on Hwy 3 I was surprised to see a lot of downed aspens and cottonwoods. Apparently an early web snow storm a few weeks earlier had downed a lot of trees that still had leaves on them.
3, Crowsnest through Sparwood, Fernie and points east is nice. I haven't tried to cross the first set of mountains on the BC by backroads, though I have camped at Whitesawn Lake and visited a couple of the hot springs in the area
3 from Cranbrook to Creston is relatively low level highway. 3 east of Creston crosses the highest allseason paved pass in BC. 3A north along Kootenay Lake is nice, though when I drove it, views were obsucred smoke from grass fires near Creston.
Alternative backroads route is Kimberley to Gray Creek over Gray Creek Pass. You can even buy a before-and-after T shirt at the general store in Gray Creek.
There are number of alternative roads in the Nelson, Castlegar, Rossland area. Supposedly there is even a difficult backroads way from South Slocan area to Fauquier on Lower Arrow Lake.
I tried a historic Cascade route from Christina Lake to Rossland. The first 20km from Christina Lake was straightforward forestry main to the pass. The next leg had 'closed' signs but no indication as to where or why the blockage. Since stormy weather was approaching I only tried a few switchbacks (down towward Rossland) before returning to Christina Lake and taking 3 over Paulson bridge to Castlegar and Nelson.
In the Grand Forks - Greenwood area you can take a gravel road past the historic Phoneix mining area. Apparently forestry mainline roads north from Grand Forks to Neeles are quite passable, though long and dusty.
Kettle Valley to the Okanagan is another area where you can take forestry roads instead of 3. Oliver to Cawston is another gravel alternative to 3.
I've take a steep gravel road up from 3 at Hedley and on to the west side of Apex Ski area. Princeton west to Coalmont and Otter Lake is paved but with some scenic canyon driving. I'm not sure how feasible it is go from there to Hope (in effect cutting around the north side of Manning). But the drive through Manning is nice, especially if you include the side trip up the mountain just north of the lodge.
A whole nother option to hwy 3 is to drop into Washington and take backroads - some paved and some gravel - through towns like Northport, Orient, Curlew and Oroville, returning to Canada at Nighthawk (day light hours only).
Four Wheeling in the BC Interior by Mark Bostwick covers backroads from the Kootenays to Cawston, as well as routes to north in the Cariboo and Chilcotin.