If you can, use arugula leaves from the garden or farmers' market; they are more peppery. Nasturtium blossoms and leaves are both edible. The flowers can be found at farmers' markets and specialty-food stores.
Look for locally grown berries that are brightly colored and plump. They should have the green caps attached and be uniform in size. Avoid soft, shriveled, or moldy fruits. If the strawberries smell sweet, they will most likely taste sweet, too.