The Pipecleaner (aka "Fuzzy Sticks") Challenge is one of my favorite STEM activities to introduce students to STEM learning. This is a fun, inexpensive, and engaging challenge that can be tailored to different ages, settings, and time-frames. This challenge is also an excellent team icebreaker.
Saving Sam -- Partners must save Sam by retrieving the life jacket from under the boat and putting it on Sam. Groups may use only paperclips. Kids cannot touch Sam, the boat, or the life preserver with their bare hands.