Engineering is Elementary has produced a free afterschool middle school engineering curriculum that can be accessed at www.eie.org/engineering-everywhere.
Although designed with afterschool in mind, the resources could be used during the school day. The curriculum is aligned with the Next-Generation Science Standards (NGSS). Although engineering is the focus (solving problems in the world around us), it does this in science context consistent with NGSS, so there are more than just engineering resources available.
Hi- These units are free. Some come with interesting videos to set the context. Same high quality as the award winning EiE curriculum but for out of school use.
Here is a green engineering unit for grades 6-8 http://eie.org/engineering-everywhere/curriculum-units/here-comes-sun
Here is one on mechanical engineering involving for capturing a dangerous frog for grades 3-5 http://eie.org/engineering-adventures/curriculum-units/hop-it