Bgcall is a small and simple tool which can regularly change your wallpaper, and optionally add a monthly calendar to your desktop.
This isn't one of those tools that will find and download new images for you, so you'll need to have a good collection of wallpaper pictures already, but if that's the case then there's plenty to like here. Bgcall can quickly import your chosen images into up to 8 different lists, and reorder them to suit your needs. It can then automatically change your wallpaper once a day, or every time your system starts. And you can also launch a special shortcut to cycle the wallpaper whenever you like.
There's also the option to further customise your desktop with a simple text calendar of the current month. This is extremely customisable, so you can set the calendar position, font, date format, text colours and more. You're even able to mark particular days as "holidays" (although these can also be birthdays, anniversaries or anything you like), and these can then be displayed in a different colour.
And while none of this is particularly revolutionary, Bgcall at least manages to do its work while making very few demands on your system. So it's a tiny download (221KB), there's no adware, and the program doesn't need some resident process to be running all the time. Once set up, it launches at boot time, changes your wallpaper if necessary, then closes down and gets out of your way.
If there's a small issue with the program, it's that the Help file is Japanese-only. This is unlikely to be a major problem - the interface is fairly well designed, you'll probably figure out the key details yourself - but if you are confused about something then it could be a concern.