Simple project for grandkids

jackmanjls:
I'm looking for suggestions for my 11 and 12 year old grandkids (both girls).

I'd like it to be simple programming and hardware using UNO. Something that has a WOW factor.

Anyone out there have any experience or suggestions?

I've been working with UNO for some time and have accumulated considerable hardware.

Thanks.

Not an Arduino, but take a look at the BBC micro:bit http://www.microbit.org.
Buy it from Amazon, Adafruit, Sparkfun
I am thinking about getting one to try since I can program it in microPython.