Plants, either indoors or outdoors.
Plants make you happier. It seems crazy, but it's true. Some of the points in this article are that exercising outdoors boosts your mood and just spending time outside makes you better able to focus and more resilient to stress. Living near green space makes you live longer.
Having plants inside your home has all kinds of benefits as well. I've got to admit that this surprised me and tomorrow I am going to get some plants for my house and for my mom's assisted living facility room.
Psychology Today - Plants Make You Feel Better : plants have been shown to lower blood pressure, improve reaction times, increase attentiveness, improve well-being, lower levels of anxiety during recovery from surgery and more!
They filter the air of course, but they also make you happier! In this study, people were given a house plant to take care of and they reported feeling happier in general.
And, there's a bonus, they also make you smarter. Houseplants Make You Smarter.
Even cut flowers improve mood, so if you have a complete inablity to nurture a plant and keep it alive, buy yourself some flowers every now and then. :-)
Just for fun, go look at these wonderful balconies on Apartment Therapy.