Easiest way to make my backyard a relaxing retreat.

Windchimes, a small water feature , a privacy screen on both sides of the patio that helps to block out the rest of the world for just a little bit .

Potted flowers also add a nice relaxing touch.

You could add a sauna tub. They now have units that get blown up which allows you to take it down at any time...they aren't permanent.

I would have a hammock, comfortable chairs, a table to eat from, a water feature, maybe a fire pit, an arbor, bird feeders, sculptures. It depends on what you like (I for instance do not like hot tubs, but my relatives love theirs and use it often) how much space you have. How much you can afford. How much time you have to sit in your back yard. We have been working with columns lately and now I think I would love to have a plain concrete patio (square or rectangle) with four columns in the corners.

see above.

It's your retreat so pick what you like and what calms you down. Wind chimes are wonderful. If you don't have room for a hammock, they've got those giant things that you use at the beach or camping to relax on (or nap!). Maybe a fun patio umbrella and twinkle lights. Don't forget aroma therapy and music.