I'll be starting with an often overlooked plant that is really quite remarkable, the common blue violet. Both the leaves and the flowers are edible year round. Well, "edible" is an understatement. Lots of wild foods are edible, particularly in the early spring. This one is rather tasty. The crisp leaves have no bitterness while the flowers contain a mild sweetness and light crunch of texture.
Chances are you won't have to go far to find some. If you've not used broad leaf weed killers on your lawn, there may well be patches where violets are thicker than grass. Due to their small leaves, they are relatively slow to collect. It takes around 10 minutes to gather 1.5 cups of salad. Preparation is simple: rinse and season to taste with oil and vinegar.
As a hobby chemist, there is another reason to harvest violets. A strong infusion of the flowers makes a pH test solution. Adding a drop of acid to the infusion creates an immediate color change. You'll go through a lot of flowers very quickly, and I never got it consistent enough for qualitative measurements, but it is a simple parlor trick.