Some easy ideas I have used in the past (where the Elf actually does something, rather than just chucking him wherever he wasn't).
Kids' pants/socks on the tree, with the Elf "stuck" in them.
Elf "stuck" underneath a big pint glass with HELP written backwards.
Elf drew moustaches/glasses / horns
on some family pictures.(on the glass)
A row of mismatched shoes with character toys in them, Elf at the front with a sign "All aboard the shoe shoe train".
Toilet paper snowman
Elf makes snow angels with cereal
Snow ball fight... toys and cotton wool thrown about
Elf does dress up - bracelets, hair clips etc on him from DD's jewellery box.
Elf drew faces on bananas.
Elf with a box of eggs and a hammer . One egg cracked with ketchup on top, and drawn (horrified) faces on all the other eggs.
Elf tucked up in bed in the doll's house/the cat's bed.
Toothpaste message in the bathroom mirror.
Dresses up as Harry Potter. (We already had the stuff , but you can do another character)
Elf skool.. elf with whiteboard/paper and pens , surrounded by dolls.