Perhaps the most pleasing aspect of Calvin and Hobbes
is that Watterson
takes a very dim and vague view of reality.
The only time in the strip where there is no element of fantasy is when something really bad has happened... And I think I have seen only a few such strips (such as when Calvin lost Hobbes).
For instance, take Hobbes. Those who say he's a stuffed tiger who comes to life really miss the point. It's more a matter of, and I'm paraphrasing Watterson here, Calvin seeing Hobbes one way, and everyone else seeing Hobbes a different way.
And this theme is explored throughout the series. Very often, humor comes because of the conflict between Calvin's worldview and his parent's worldview - he may see aliens, octopi, and other strange creatures where his parents see some snow, or rocks, or even nothing. Calvin's snow sculptures are definitely outside the realms of plausibility, but it doesn't matter in the strip, because the strip is not concerned with reality, except as it appears in the eye of the beholder.
This is, for me, one of the main charms of the strip - that it shows different world views co-existing, often in fun, although not necessarily funny, ways. Calvin and Hobbes was not great because it was funny, but because it was fun, engaging, unique and memorable.