Here is some material to read in German.
http://www.childrensbooksforever.com/childrenpages/German1.html A collection of German kids' books. Since they are written for small children, they are dead easy to read. There are books here in other languages, too, if you are looking for learning material in other languages.
https://ebooknotifications.wordpress.com/2015/05/15/a-free-choose-your-own-story-book-for-german-learners-and-finally-part-iii-of-the-aschkalon-trilogy/ This is a choose your own adventure book written for German learners. Normally it costs money, but right now, it's free! It's a really fun read that isn't too difficult, especially considering that every chapter ends with a summary of difficult words. (The way I wrote this makes it sounds like an infomercial. I'm not paid to advertise this, I swear :'D)
http://www.wasistwas.de/ This site has many articles on various topics. The difficulty level is not too high; if you've finished the German tree, you might be able to read one of the most complex articles with the help of a dictionary. I've successfully used it to revise for history tests and practice German at the same time.
http://www.goethe.de/ins/gb/lon/prj/asa/top/enindex.htm Here you can find many articles and websites, intended for those at an A2 reading level. However, some of the links do not work correctly - watch out for that. The overall difficulty seems to be greater than the text from the previous website; try to read these if you like a challenge.
That's all I have collected that isn't already very widely known (such as the dw.de slowly read news) :D Feel free to add in the comments if you have any other good websites for reading material.
For the daily news read slowly. Its a great daily thing to stay fresh.
There is also a German VCE Exam to test yourself in your reading, writing and listening skills!! Those may be challenging for some but it expands your vocabulary!!
Here is also some really good content.