This free teacher eBook is a nice publication to browse, courtesy of NEA.
You will find all sorts of links and resources: “videos, games, posters, lesson plans, books, online courses—all in an annotated list that spans multiple topics and grade levels. A wealth of free stuff for your classroom is just a few mouse clicks away.”
I browsed through the book and found lesson and activity ideas for K-12. Take a look – there is something for every grade across the curriculum spectrum.