Even though I grew up with a house full of animals, I didn't know until somewhat recently that many foods and plants were toxic to cats. I knew the obvious ones, like lilies, but I had no idea there was such a long list.
We ended up planting them a pot of cat grass on one deck, and a cat-friendly herb garden on the other -- mint, parsley, basil, lavender, catnip and mini roses (rosa species). We don't let them out on the herb garden balcony often because they love catnip a bit too much (and really this is the only plant other than the grass that they are drawn to), but they do enjoy a bite of grass from the plant on the other deck from time to time. Especially Max who would probably be a vegetarian if we would let him.