I have directly sown lettuces, arugula, spinach, rabe, purple top turnips, daikon and watermelon radishes, bok choy, mustards, and Asian greens. I started kolrabi, kales, broccoli, cauliflower, and cabbage in flats. I have to, annoyingly, make another raised bed because I have no room for my brassicas.
I planted around 100 carrot seeds, put a floating row cover over them, and watered everyday, but much like that children's book, nothing came up. and nothing came up.
I don't know what the rule is for plugging things, but if you go to "one yard revolution", he has a fall planting excel sheet that you can enter your first frost date and it tells you when to start your seeds.