Running works great for me for several reasons:

1) It's got a fairly flat learning curve.

2) The rewards are immediate and gradual, but clearly noticeable.

3) It's relatively cheap, though you can't cheap out on shoes if you are going to run on cement. A good pair usually costs over $100, but you can get them for half the price online if you know your size for a particular model.

4) It's convenient. However, you do need access to a shower afterwards.

5) You can do it alone or with someone else, at any time you want.

That being said, I found taichi to work just as well in clearing the brain, but you don't get the endorphin rush or the cardiovascular improvements as much.

It's FAST too, in terms of calories burned per minute.

I love cycling - but any bike ride under an hour isn't really worth the time, unless you're doing hard intervals or climbing a really steep hill. If you have 30 minutes, there's nothing more efficient than running.

Tai Ji is incredible in my experience, but it does take a lot of initial investment.

I'm afraid hang gliding would clear out my brain a little too literally.

I'm sure many people will disagree, but long-term I found running absolutely wreaked havoc on my knees. And studies I've read say that modern shoes are part of the problem.

I enjoy yoga but it always leaves me wanting more.

To each their own, I guess.

I'd like to recommend aviation in general. Nothing like aerobatics or instrument approaches to clear the mind.