With regard to books, it's good to read some books (like Zen and the Art of Motorcycle Maintenance) when you are young, and to then dip back into them 10 or 15 years down the track. I've been interested to see how differently I perceive, or how differently I savour and enjoy, certain books now. Some books have more meaning for me, some have less.
With regard to comments about exercise, when I was younger I did a shed-load of cycling, mostly off-road, but my bike was also my main means of transport. I'd head out on the bike at least five days a week and sometimes stay out for half-a-day or more. Thankfully, there wasn't Instagram or Tumblr back then, or I might have felt the need to obsessively post selfies about my cycling, like a well-known former SF identity! I also played a lot of squash, which is a great game - fast, technical, more complex than tennis and really good for fitness. Whenever I was feeling a bit down, I'd either jump on the bike and head out into the forest to clear my head, or I'd grab a friend and play squash for an hour. As Nolvadex said, after an hour of exercise like that, your body feels tired, but your head feels great - you're exhausted but exultant.
Anyway, in the vein of PoP's comment and the other rather zen-like comments above, I'll add this:
It's the "tsukubai", or water basin, at Ryoanji temple in Kyoto, Japan.
The characters around the outside, all read in conjunction with the receptacle for the water in the middle, which forms part of each character, reads "吾, 唯, 足, 知", or "Ware tada taru (o) shiru". Although when translated literally it means "I only know plenty" the philosophical meaning of the inscription is "What I have is all I need", which fits in with the basic anti-materialistic, self-denial tenets of Buddhism.