I love the dorky part of Sheppard too, and overly-serious fics (unless the situation demands serious Sheppard for the duration) feel OOC to me too.

It's interesting that's the spin you put on the flying; I can totally see that (and I know I've read some fantastic fics that characterize him that way), but it's not my go-to rationalization with the flying (I think melannen hits more on the reasoning for me below, with flying = becoming free of whatever is holding him back and Atlantis = the thing that is home that doesn't hold him back). It makes perfect sense, though, and interestingly, for NC17 stuff, I can totally see that characterization with regard to kinkiness (and thus why sub!John makes sense to me but top!John less sense (and when top!John works for me, it draws from the more serious side of his characterization)).

And for all that - I still do usually replace flying with racecars, bungee-jumping, horse riding, things that are exactly what you describe above.