It’s a fairly hidden feature that not many people seem to know about, but the Apple TV has some pretty great options for customizing its screen saver. There are a few default sources of photos you can choose from, including National Geographic (probably what you are using now), Animals, Flowers, Trailers (which shows movie posters of films on the iTunes Trailers website) and iCloud Photos (Photostream and iCloud albums).
But more interesting is the option to use Flickr. That may sound odd, but the reason I say it is because when you combine it with the awesome power of IFTTT, you can create some really unique screen saver options. For example, for the last few months I’ve been using a combination of Flickr, IFTTT and Instagram to create an Apple TV screen saver that cycles through images that I have liked on Instagram and it is far better than seeing the same old National Geographic photos (as great as they are) over and over again.