This is the absolute easiest question. If you ever get this, the answer is steps. Get your 10k steps a day, or even 20k if you're feeling frisky. Get in shape, buy some nice walking or running shoes, and invest the massive difference between walking and what breathing would get you.
Once you reach a good level invested, you're functionally able to retire and never even take another step. Your investments will cover everything.
Anyone who says breath doesn't realize the sheer difference steps can make.
As your cardiovascular system improves, you typically take fewer, larger breaths at rest, so improving your ability to breath reduces the number of steps you'll typically take. On the other hand, most people become able to walk more steps after walking consistently.
Unless you're handicapped, bedridden, very old, or otherwise prevented by health related circumstances from doing so, you can make more with steps.
If you can't take advantage of the huge increase in income steps have over breaths, then that's more of a budgeting problem. It's incredibly easy to pay off your home and have a huge chunk of emergency funds available even if you get injured if you average 5000+ dollars a day with steps.