I used to do this annually in my 2nd grade class. It was a big deal!
I gave them a project to do--collect 100 objects (buttons, cereal, paper clips, etc) and display them in a creative way to be exhibited in the classroom. They had fun with this and some of the displays were very good. One I always remember is a dolphin made out of buttons.
I also made up centers with different activities based on 100.
--How tall a tower can you build with 100 toothpicks?
--What can you make with 100 legos?
--How long does it take to put a 100-piece puzzle together?
--Make a necklace with 100 Cheerios (or Froot Loops, for color)
--Decorate "100" glasses
--Make a headband with 100 tally marks. Then circle groups of ten.
--Play a game "Race to 100" using a 100s board and dice. Add up the two numbers on the two dice and color in that amount of blocks on your 100s board, adding each turn. The first one to 100 wins!
That's what I can remember off the top of my head. I haven't taught 2nd grade in 3 years. My memory is starting to fail with everything new that I have to do in 4th grade.
Good Luck! Have FUN!!
