I purchased several different kinds of building materials for my kids. I also went through my stash I already had at school. I put each material in a different plastic container with a lid for easy storage and clean up.
When students finish up their work, they are allowed to choose a STEM bin. They can build anything they want (as long as it's school appropriate.) If they need ideas, I found some great STEM task cards on TpT. I had to put a limit to no more than 2 students per STEM bin to help with noise. They can get a little loud while they are creating! Here are some of the amazing things they made just before our Christmas break.