A word for "positive side effect"

You can use the words : Benefit, advantage, feature, plus point and more..

I suggest you get rid of the "positive side effect" verbiage and replace it with one word: moreover.

Planting a tree will help the environment; moreover, you will have a nice decoration in your backyard.

Or, with a few more words:

By planting a tree you will not only help the environment, but you will also have a nice decoration in your backyard.

Or, simpler yet:

Planting a tree helps the environment and adds value to your backyard.

I like this one. It has good balance. You've got one action with two results. Planting does two things: it helps and it adds value.