Kohlrabi is delicious raw! I peel it and eat it like an apple. (I also do the same to, uh, turnips. Because I'm weird.) Maybe you'd enjoy it peeled and shredded and served in a big salad with the lettuce? To me it tastes almost exactly the same as broccoli stems, which I happen to love, so anything you'd do with broccoli would work with kohlrabi - steaming, stir-frying, etc.
My favorite way to cook most tender greens is to heat a wok, put a bit of oil in it, maybe a tablespoon, wait for it to come up to heat, and put a bit of fresh-ground pepper and red pepper flakes in the oil for just a few seconds, just long enough for it to become fragrant, not so long that it starts to smoke (if you'd like to add some slivered garlic, this is a good place to do it as well). Then add in the chard. A lot of recipes tell you to cook the stems and the leaves separately: I usually don't bother unless I'm feeling ambitious. Instead I just rinse and chop the entire bunch and toss the greens and stems in the hot oil. They'll wilt almost immediately. Salt if you like. You can finish it with a drizzle of vinegar (rice wine is good), or lemon juice. And a drizzle of sesame oil is delicious, too.
Under a minute from cutting board to plate. And so yummy. I'm so jealous of your box of chard, LOL!
For kale, I either do the same as above (although it takes a bit longer, and I might put a lid on the pan and lower the heat so it can steam in its own juices for a minute or two), OR I make oven-baked kale chips.
Turn the oven to about 350. Wash and dry the kale and tear or chop into pieces (again, you can take out the stems, but I'm lazy.) Toss with a tablespoon of olive oil and some salt and pepper, spread into a single layer on a cookie sheet.
Bake, keeping a close watch, for anywhere from 5-10 minutes. They're done when they've become crispy, but aren't brown.
Really, really good. And I like to pretend that they're super-healthy.
For lemon balm, you might make an herb iced tea? It's very fresh-tasting and would be good in hot weather.
For sorrel, this recipe
looks interesting (although I haven't tried it). Or I would just make a simple soup: wilt a bunch of sorrel (and you could even throw in some lettuce, if you can't eat it all in salads!) in some butter or oil, pour in some broth, puree, finish with a bit of cream if you eat it.