If you have a small balcony or a small terrace than it probably is already overflowed with plants. The best way to add some more is to organize some plants vertically. Nowadays you can buy systems to organize vertical gardens from different manufacturers but who want to spend money if there is the possibility not to. For example you can use a wood pallet as a platform for your vertical garden. This way such garden will be almost free for you.
Fern Richardson from Life on the Balcony has done such vertical garden and showed us how. She used a pallet, 2 large bags of potting soil, 16 six packs of annual flowers, a small roll of landscape fabric, a staple gun, staples, and sand paper. Check out her blog for more photos and full instructions.