Every year my former STEM program put together a giant kickoff for our students. These events were designed to promote collaboration, trust, and community while getting to explore some awesome STEM ideas. One year we themed the entire kickoff around sustainability as our students would be engineering with repurposed materials in their design class. Another year, we had groups design boats out of cardboard and then we tested them out in a pool at the end of the day.
Last year, we decided to go with a more diverse plan. Each teacher designed their own mini-PBL and students were able to select which they wanted to attend. It reminded me of the excitement of going to a big state or national conference- the hardest part was deciding what you should skip! This gave us all a chance to nerd out a little in our respective subject areas and do things we might not normally have time for with a diverse collection of grades and ability levels.
I settled on using the time to have my kids explore Balloon Geometry. Besides spending two hours before school blowing up balloons and having incessant popping sounds coming from your classroom, it was fun & a great engineering challenge. I saw groups work together to problem solve, play to each other's strengths, and just have fun thinking critically about geometry.
Here are my slides: Balloon Geometry
You could make this way more structured to ensure that kids are building "pretty looking" polyhedra, but this was only an hour activity and I liked seeing the kids work together to decide what figures would work the best.
There are TONS of great resources online for this, but here are a fun of the ones I used: