There are some really nice additions you can make to salad to give it a little extra punch. My two favorites are watercress and nasturtiums.
Watercress is a highly nutrient-dense green (up there with kale) that I usually pair with other greens to mellow out the spice. The spice of watercress has a similar zing to radishes. Often times I will use one part watercress to three parts spinach, baby greens, or arugula. Watercress also goes nicely laid across fish or mixed into soup. To store, be sure to put the root ends into water- this will help it keep for longer. There are other varieties of cresses out there; we grew curly cress on the farm I worked on.
Nasturtiums are another spicy addition to a salad.
They are beautiful flowers and are quite easy to grow from seed. I will pick nasturtium flowers off just before putting on top of a salad. Not only do they make it taste delish, they look so pretty and it is a really nice detail in a salad. Everyone likes to eat pretty salad.