Pointless pursuit
Put a marble on the edge of a big desk. Use tape to mark near the corners. Pupils have to roll the marble with their nose around the table as many times as you tell them. The count resets if they don't pass a corner or it falls off the table.

Gum Chum
Give pupils rubber gloves, a face mask and some goggles. Give them a set of scales from science and tell them they can leave when they have collected 100grams of chewing gum from under tables.  This is actually about 80 pieces of gum (I have the photos to prove it)

Loose Leaves
Send pupils to collect 10 different leaf types. Give pupils a piece of A3 paper and some sellotape and scissors. They have to cut the leaves into strips and stick them doen into the words ' sorry I have not brought you my homework Mr / Mrs delete as applicable, and add a border, all in bits of leaves.  (I have the photos to prove this is possible within 45 minutes)