Make Doctors appointments for embarassing illnesses in your friends names. And give their work number as the contact number.
When someone drops litter, follow them home , find their address, and mail their litter back to them. Without a stamp.
If you don't like your boss, report him to the police as a terrorist or similar, and get the to confiscate his computer. For added effect, break into his house and place files on his computer called "how to make bombs" etc.
Hold the door for people, and then follow them pestering them for a tip.
Fill a trolly full of shopping you don't need, and then when you are buying it at the till, suddenly take an urgent phone call, and say you'll be back in a moment (run to your car). Then don't come back.
Order taxis from 10, 20 etc different companies to the same address (Balls of Steel.)
Join an online dating site as a "fit bird", then get as many mobile numbers as you can, and exchange them between different blokes, pretending it's your number. Either you get a whole network of people all texting the same poor guy, or pairs of people texting each other, thinking the other person they are texting is a girl.
Same as above, but arrange a date online, and don't turn up. (Obviously- because you don't exist.)
Even better- arrange a date with several people, at the same location, and don't turn up. See if they realise. Give out half their phone numbers to other half os the people on the "date" , saying call me at 8:10 if I'm late, I'll be on the bus" so as to produce a random chorus of rings at the location.
Hand out politically moivated leaflets targeting an organisation that doesn't exist. " The RRFG Group is polluting our countryside, and must be stopped."
Throw fireworks at cars.
Throw cars at fireworks (bit harder.)
Ring up a brothel and ask how much they would pay you.
Support Mancehster United and not be from Manchester.