Eat healthy,drink water and exercise,who said sex is healthy.....
What Is The Healthiest Thing That You Do For Yourself (other Than Sex)?
My favourite thing is practising Kungfu. It keeps me healthy and clear-headed. My idol is bruce lee.
The S word is very healthy under the proper circumstances, when one is married, but since that is not an option at this point I agree water is great but too much can be harmful as it will deplete vitamins in your body so 8 to 12 glasses per day depending on your life style and size. Walking, running is very hard on joints, mind games such as puzzles, flying kites, reading and playing here we go loop da loop. Oh yes, praying to your maker and doing good for others is very healthy for your spirit, soul and psyche. If you want what I consider to be the healthiest of all, I can't answer that. 
Exercises.
Early in the morning I have 1 glass of Lemon juice (without sugar/honey or salt)Having healthy food like having leafy vegetable curry in my lunch and dinner. In between taking fruits and last but not the least have lots and lots of water daily
Frequent exercise and drinking water and poweraide. And eating some type of pasta after a work out helps out tons.