It is usually the smell of certain plants that I find soothing. I see you have a picture of lavender in your question, this is definitely a soothing plant - lavender oil is very good for stress and headaches.
Balms are very good for soothing the nerves too - I love the smell of lemon balm.
And then there are those we use in teas, such as chamomile or mint.
There are also certain colours that are soothing to the eye, so I suppose there must be some that are soothing just to look at too.