We went today at 12:30 and it was crowded, but not oppressively so. You have to change your money to tokens for the food booths. There's a little token hut between the vendors and the tables. There is side-street parking available a block or so south of the temple, away from Roscoe. The mango with sticky rice and kanom krok are indeed the stars of the show, though I really liked the grilled chicken skewers as well. It's a fun experience.
Other than the "best foods" question, that post doesn't answer yum's enquiries.
It is outside, on the north side of the temple. You don't have to pay to go, but you might consider donating to the temple, and/or paying your respects inside (remember to dress modestly and take off your shoes before entering).
Either day is fine, I find Sundays a bit more crowded.
I love the papaya salad, I love the mango with sticky rice, I love the kanom krok (the coconut half-moons).