We travel between the NW US and our cabin in NW BC every year. Often different routes. Last year stayed at Kleanza Prov. Park near Terrace - very nice spot (we were there in early May), but may be busy midsummer. Most of the provincial parks are pretty nice and if you prefer a little more rustic, there are a number of what are called recreation sites that are smaller, less used places. Smithers is a good supply point (and a nice town) on the Yellowhead hwy. If you want to get off the main road, Ft. St. James is kind of interesting - mostly logging/paper town but lots of nice spots to explore around there on logging roads, if that's of interest.
Another interesting trip would be highway 20 west out of Williams Lake to the coast. Not highly used (though it has been 10 years since I was on it last so no guarantees), interesting scenery particularly at the western part in the higher mountains. Plan on several days minimum to do it justice.
As already mentioned, highway 37A west from Meziadin to Stewart and Hyder is beautiful. Gorgeous coastal country on the old mining roads north out of Hyder up toward Salmon Glacier and the old Granduc mine area. And the Cassiar (37) north toward Yukon is a nice drive although they just put in a huge power line up the first couple hundred miles. Quite an eyesore, in my opinion - it could have been done with more thought toward the visual effects.
Just a few things to consider.