There is nothing like the sound of flowing water to sooth your senses.  I recommend some kind of a water feature near a place where you sit.  Even if it is only a water fountain, the sound of the water splashing adds so much depth to your outdoor experience.  Of course a full sized pond with aquatic plant and animal life adds layers to the experience of spending time in nature.  This is a Koi pond with a shallow beach area protected from the larger pond with stones and aquatic plants for baby fish and frogs.

Streams and waterfalls are another option to bring the sound of water play into your environment, and invite different creatures into you natural world.

This was taken in the fall to show you how important static elements like sculptures are in the landscape.  How much better if they serve a dual purpose and bring sound and water into the environment!