There are many herbs that cross over into the categories of edible flowers and salad leaves. The flowers can be picked and added to drinks, salads, cakes and breads; just be aware that many of them have the flavour of the herb - so garlic chive and rocket flowers may not be ideal for a cake. The flowers may also be crystallised eg Heartsease flowers for cake decorating, so get creative and colourful with your food!
In general the herbs that you pick for salad leaves are perennial and hardy so pick them as they get large enough or cut back hard and wait for the new growth to appear for new and soft leaf growth. The most popular ones are probably Rocket and Sorrel but any of the fleshier edible herbs may also be used.