I don't know why local-America sells "spring mix" but not "winter mix." Here are some things you can grow or buy to make your own winter mix.
However, do keep an eye out for the "bright herb" versions of spring mix. These add things like basil and thyme to the usual spring mix of lettuces and baby spinach. The effect is similar to what I get by picking a bowl of random herbs and throwing them into the plain spring mix.
If you like brightly colored food, consider adding edible flowers to your salad. Last year's effort was such a success that I got asked to do it again. If you enjoy onions in your salad, definitely try regular (pink) or garlic (white) chive flowers. They are small, crunchy, pretty, and pungent.