A few fun salad greens to consider...
Lemon Gem Marigold: The word lemon is in the name but the lemon is not so prominent in flavor. They taste like a mix of bitter grapefruit pith along with a slight citrus freshness-along with that taste of greens. You can separate the flower and just use the petals but I think the most interesting flavor is in the whole flower. And the leaves may be the best part, very nice mixed into a salad. Garnish with a few flowers for a nice touch.
Lemon Balm: As the name states, it's lemony and fresh at first with a strong ending of bitterness. This would be nice used sparingly along with a roasted piece of fish on the side.
Salad Burnet: A little reminiscent of cucumber skin...You want to just use the actual leaves with this one, as the stems are a little tough and you want to use the mature leaves as the little clusters of new growth are bitter. (Not enjoyably so.)
Nasturtium Leaves: These are nice while you are waiting patiently for the flowers to bloom and are craving a little peppery and bitter bite to your salad. They are beautiful and striking in a salad.
Corsican Mint: If you are a mint lover, you will love these ever so tiny leaves, as they are so concentrated in mint flavor. They are so pretty and delicate and would be just gorgeous strewn around the outside of a plate as a garnish.
Lemon Verbena: A great lemon-lime flavored leaf, that's not overwhelming and would be pleasant in a salad with more of it, if you are a fan of lemons and limes.