While there are many games that are relaxing to play, The Garden is unusual in that it seems actually designed to relax you. It's relatively short, at about 15 minutes, and requires little more from you than sleepy management of your "mana" and "focus".
You gain mana over time, and as your garden grows, you gain it more quickly. You need mana to paint your garden, and to increase the detail in your garden you need focus. Focus also costs mana, so you have to choose where you want to use your mana as you gain it. The the game starts with a mottled gray screen, and you are encouraged to paint it. Over the 15 minutes or so of play, the screen gradually gains color and detail.
When you have finished, it's game over. There's nothing really challenging about The Garden, it's just a pleasant experience. Learning what to do feels relatively organic, with just a little encouragement from the game as you start.
It's short, but the Garden is free and a very relaxing little experience.