We had a little thing here a while back in another thread on this topic, with a newbie asking about ideas for soloing over "So What". I offered the ideas of listening to Miles' solo, of trying to use relatively few notes to play with the space between notes, to try rhythmic variation, and to try a bit of blues.
Personally, I like the sound of some flat 5's in "So What" and have played them without shame. It's a little bit of earthy blue tone in there and it shocks a bit by being outside dorian. There are some around here, though, who opine that if ain't dorian, it doesn't belong in "So What".
My own opinion is that Miles would have scoffed at such "rules", especially over 40 years after the fact, but what do I know?
I agree fully, BTW, that the tunes are easy to pick up but "difficult to play well." Depending on your definition of "well", "Happy Birthday" can be a challenge.
I still say it's a great tune for getting newcomers used to the idea and the feel of improvising over simple changes. They won't sound like Trane or Wayne Shorter with their musical ideas, but they're learning!