Technically it is treason to put a stamp on a letter upside down - how about sending a postcard or a letter with the stamp upside down?
Love the shopping list seeding.
How about sitting in your front garden (if you have one) or doing something naked that you normally do clothed.
Go to a library and move some books or change the label on them if they have bar codes, so that someone takes out an enid blyton but it scans as 50 shades.
If you do the seeds spell out rude words on someone's lawn.
One I saw on here once, go to someone's house (preferably someone you don't like) and move all furniture 1 in tot he left.
Make something up about game of thrones and put it on facebook, say an episode has been leaked.
Oh - one I did to a friend. I asked him to check my bathroom because I thought there was a leak.
Sitting in the bathtub was an actual leek.
You could do this is a supermarket toilet, just leave a leek for someone to report to staff.
Go buy a 'murder kit', gloves, knife, hacksaw, rope, bin bags, at the checkout ask if they know a good place to dispose of a body because the old one has run out of room.
Wet some £5 notes and hang them on a washing line in the hallway / front window. Order a take away and when it arrives check which money is dry, unclip it from washing line and hand over.