I think I have to see his dorky side for me to believe it's Sheppard. Interestingly, in Harlequin-type stories where Sheppard is The Guy, this part is almost always left out, so the characterization feels flat and OOC.

If he doesn't fly, he has to do something that gives him, hm, an adrenaline-fueled sense of peace. In a couple stories it's riding a bike, which I believe. Climbing would be another thing, or certain kinds of music. Horse riding -- any kind of vehicle or conveyance, really.

Here's the core: John loves activities where you push really, really hard and fast, and then there's a moment when you cut free (mentally, emotionally, physically) and glide on high and untrammeled. That, for him, is the core experience of life.