The thing with mizuna is that it is amazingly hardy. Some had self-seeded outside and grew through all the deep snow we had last winter. The outside stuff did go to seed quickly though. But as soon as you see the flower buds appearing, cut them off above the lowest leaves and eat them - they are all edible. That then makes the plant bushier so more leaves to eat. You can go on cropping until the whole plant starts to get coarse. Before then sow another lot which will be ready when the first lot goes in the compost heap.
As well as looseleaf salads I like to grow very small lettuce types. My favourite is Mini Green. It takes as long to grow as a full-sized crisphead lettuce, but is small enough for a meal for one, and much tastier than Little Gem, although I like those too. I will be sowing some lettuce soon, even up here, just in case it succeeds and I will have an early crop. If it gets frosted, well, the seeds are not expensive.
Another salad crop I enjoy is landcress, which is like watercress but doesn't have to be grown in water. It's nice and spicey.
Radish can be sown in a few weeks time, or you could try some now with good protection. If it succeeds will depend on the weather.
You can also grow sprouting seeds indoors, to give a tiny amount of fresh greenery in the dark days - almost anything can be grown like that, including cress, brassicas, peas, mung beans, etc.