There’s a partial solar eclipse coming up - partial in the UK anyway - which should be detectable using a light-dependent resistor and a handy 8 bit micro.
I did this (also measuring temperature and humidity) with my son for the 2017 US eclipse. We visited a friend who lived in the zone of totality and ran the arduino spitting data to a logging computer and could even see in the data when thin cirrus clouds passed across the partially-obscured sun.