Country_Wife, I agree with you that 4+ hours of driving in one direction is too much for a day trip. Interestingly enough by that definition Victoria, which is such a popular day trip out of Vancouver, only JUST qualifies as a day trip. I personally do not think it is ideal to do Victoria as a day trip, but zillions of people do it.
I personally find that 5 - 6 hours of driving, in total, is feasible. It depends on what else you’ll be doing with that day. Obviously if you drive for 5 – 6 hours, you cannot also do an extensive hike.
My definition of a day trip is an 8-hour outing, that is, an outing in which the driving and the other elements together add up to no more than 8 hours. If I’m responding to a query on the forum and I recommend a more ambitious outing, say one that will take 9 - 10 hours in total, I try to remember to include a warning to that effect.
I don't know what you mean when you say that a day trip should not involve plane travel. If you are referring to conventional plane travel, the kind that involves going through airport security, etc., I think you're right. However, I think the floatplanes that go from Vancouver Harbour to Victoria Harbour are fine for a day trip (although there again I try to remember to include a warning that they are more expensive than surface travel).