Put on a performance for a CCTV controller. You could improvise a short play, show off a dance routine or hold a full blown talent show.
Make a giant sign.
Start at one end of your local high street and find the letter 'A' in a shop name, and then 'B', until you find 'Z'.
Travel 100 metres without being seen.
Climb to the top of the world's highest places without leaving home.
Put identical containers outside in different places when it's raining... does one fill more than the other?
Paint a stone red. Leave it somewhere and then come back at a later date to see if you can find it again.
Cut eyeholes in a newspaper and then secretly follow some friends.How long can you go without being seen?
Explore your local area looking for signs and words that command you to do things. If you find one, do what it says.
Post paper blog entries on your street.