Summer Planting: Kale, Chard, Spinach; Mint

One thing I need to get better about is the realization that planting in a vegetable garden doesn't happen just once, in the spring. It can and should be a season-long activity, just like weeding, but better.

Today, in the place of the previous sugar snap pea stand, I "directly sowed" in four kale "pits" ~2 ft apart in a zig-zag formating. In the spaces between, I planted 2 rows of spinach and 2 rows of swiss chard. I confirmed that at least one other Midwest gardener plants kale in July--the reason being that kale likes to be alive in Spring and Fall, it can survive temperatures down to 10-below (supposedly), and frost makes the leaves taste sweeter. Sounds pretty perfect for Ohio. My friend Allison also lovingly describes kale as the "toothbrush of the colon." What can be better?
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Also, I've been regretting not planting any mint early this year. It's got a ba-jillion uses. Long story short, I was at the grocery store and found a small mint plant in soil in a 4" tall plastic container. The grocery store sells a few select planted herbs (basil, cilantro, parsley, mint). They are pretty small plants with up to maybe a dozen leaves. I'm not sure if the intention is to provide fresh herbs for a single meal, such that the container and stem are disposed after the single use; or if the plants are to be kept in a kitchen window to grow and be used as many times as possible... Anyways, I bought the plant for $2.99 and planted it in a bigger container using soil, potting soil, and a little bit of "plant-food" fertilizer pellets. I pinched a couple of the stems just above pairs of leaves, hoping to spur new growth in both the shoots (stems) and roots. Hopefully I'll be able to use mint in the next few weeks.
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I'm considering adding a few elements to the garden to extend our growing season. I've been reading about people using "hoops" (think metal hula-hoop halves placed in the garden) over which they can put shade cloth to keep things like lettuce cool in the summer, or clear plastic to keep things warm in the cold and frosty months. Another useful resource might be a cold frame, basically a box made of wood and glass which similarly protects plants from frost in cold months. I've got a couple months to consider these things...