On the 50th day I do everything by HALVES. It seems mean at times, but depending on the class, they get a kick of it and the concept of half is sure to be taught in one day (or in half a day!).
I start the day by writing HALF on the board and discussing it. I then tell them they are to only wear one shoe all day (whilst inside) They laugh and love it after a while!! It is actually funny to see them walking and working with one shoe on.
Wherever I can, I cut everything into halves. I have pencils ready that they'll use cut on the day- cut in half, I cut fruit and veg like apples in half for them to eat, I even cut time in half so when I know they are not finished something, I say stop and hand it in...
Some other examples are:
I get them to draw half a picture and I teach about symmetry.
I give them half a jigsaw to do.
Occasionally, I cut off them when they are talking to me saying "enough!"
I give them half of anything they need like paper.
By half way in the day, they are the ones suggesting to me what they could do in half.
I hope you like these ideas. They are just meant to be fun.