We grew collards this year. They aren't hard to grow, just need some water every day or every other day depending on weather. If you don't pull the whole plant, just take the outer 3 rings of leaves, leaving at least 3 inner rings of leaves, they plant will continue to grow throughout the season. The texture of the leaves is totally different than what we get at the store, much thicker and more velvety (from lack of a better word). They also take longer to cook but, hey, I'll happily trade that off for just picked greens, rinsed off, and plopped into the pot over the packaged stuff or even the bundles you get in the store. I'm totally spoiled now.
My girlfriend grew kale this past year and she says it's not anymore work than collards, which she did last year.
Both can be blanched and frozen by the way.