Herbs are easy to plant and can easily be inserted into already landscaped yards. Herbs are generally not invasive (mint is pretty invasive), they grow quickly and are not big, so they won't take up too much space. You can buy transplants for a couple of dollars each and set them out as soon as the danger of frost has passed.
Culinary herbs like rosemary, oregano, thyme, parsley, chives, etc. are a pretty addition to the yard and can be used when cooking. Chamomile and mint (plant mint in a container because it can grow very quickly) can be planted and used to make your own tea. Lavender can be used for its beauty and scent. Even if you're not planning on selling your home, herbs make beautiful and practical landscape plants.