Borage is lovely - bit spikey if not chopped fine. Salad & drinks.
Stinging nettles (the tops of spring growth only) are lovely sauted well.
Marigold petals, in salads and baking (for colour).
The ribs of chard can be cooked separately to the green, and treated like celery, with butter.
Woody Rosemary spears (stripped) can be used as BBQ scewers.
The leafy tops of celery can be treated like celery flavoured parsley.