When I started teaching STEM science using PBL, I found out that, although there are millions of resources for "dessert projects" for middle school, there was not much in terms of the invitations to learn that I was looking for at this level. I started modifying or adapting what I found, and came up with several entry documents that I am now using in my practice. All of these have a "teacher corner" where you will find enduring understandings, essential questions and NGSS and CCSS alignment. All of them are intended for middle school, but they could easily be used across grade levels depending on the need to knows of your students. Hope you find them useful.
If you are ready to branch out on your own, I invite you to read my PBL toolkit post.
If you would like to have these resources sorted by NGSS standard, visit my LiveBinder.
Life Science PBL, by mrsgarciaserrato
Earth Science PBL, by mrsgarciaserrato
Physical Science PBL, by mrsgarciaserrato
Engineering PBL, by mrsgarciaserrato
Other PBL, by mrsgarciaserrato
If you use any of them, or if you have questions about implementing, leave a comment. I'll gladly respond.
STEM Competitions, by mrsgarciaserrato