Personal anecdote, so sample size of 1 and YMMV, but 5 months ago I started doing exercise in the mornings that has done wonders for my concentration and has helped me drop 25 pounds without changing much else in my life. It takes 15-20 minutes, 3-4 times per week and I literally don't have to get out of bed to do it.

After being frustrated by my air consumption on a scuba dive, I got interested in free diving (apnea) training. I found an iOS app called Apnea Trainer that looked promising and started doing the breathing exercises. I've gone from being able to hold my breath for just under 90 seconds to breaking 6 minutes for the first time last week. It also revved up my metabolism (like I said, I'm down 25 pounds) and I've gone from having borderline hypertension (with a family history of stroke) to having blood pressure that's almost worryingly low (usually around 100/50). And, like I mentioned above, it's been good for concentration and stress levels as well.

Wet training (in a pool or body of water) requires instruction and a partner, but dry training is pretty safe to do on your own as long as you don't push yourself beyond your limits on a regular basis.