I fondly remember digging trenches[1] in a hot summer. It was the most painful thing I ever did, but I approached levels of zen I rarely have now. Everybody is built differently, but there's something deeply good about physical exhaustion. When you know the job, you get in flow easily. When done you feel all reset, the next day you feel a bit sore but it's the good kind, you even anticipate it.

[1] And if some kind of tasks are too brutal, there's always swimming (or even qigong).