Mushishi is a strangely calming anime. It takes place in almost feudal Japan (they seem a lot more liberal and have access to some technology like microscopes and the mushishi talks about genetics in one episode) and follows Ginko, a man that can see the strange lifeforms that are all around us, called Mushi.
In the end the episodes are rarely tense, with no or almost no violence. The mushi themselves are not perceived as evil that must be killed, but as a part of the ecosystem. Unlike most mushishi (a sort of mushi hunter/doctor), Ginko, the lead character, seeks only to restore the balance between normal life and mushi life.
The anime itself takes place for only 26 episodes, all self contained, you could watch any of them in any order without losing any continuity. The manga is of course much longer and you can read it here.
The calm music and the elements of traditional Japanese life and history are most welcome for a leisurely time when you want to relax and take your mind of things.