One thing Microsoft have alway been kicks ass at is desktop themes and wallpapers.
I had my homepage as Bing for a while just because of the gorgeous daily pictures.
Here is a great one, Remix 3D Animals, from the Remix3D team.
Or how about Paint 3D Creatures?
These are delivered through the Windows Store, take a look at a bunch of others here.
The Xbox Team can also serve up wallpapers in various different sizes for your device.
Halo 5 anyone?
Check out the range of games over on the xbox site here