As my classes pursue inventions during our genius hour each week, I keep coming back to the makezine website for fantastic ideas. Here is something inexpensive that beginners can do using coffee cans and the Aurdino - it does require some soldering so you might want to reserve that part for an adult unless you have older kids capable of it.
Next week on 9/11 is the 9/11 day of service that many are commemorating. You can go to www.911day.org to see what others are doing and join in. This is a great activity and focus on others but more importantly - it is a remembrance - it isn't enough but it is a remembrance. Some of us still feel like it was yesterday.