Freeze small toys in tubs of water.  Use yoghurt pots or icecream tubs - the bigger the tub the longer it will last and more children can join in together (fill the tubs about 3/4 full as the water will expand as it freezes). Try toys that will float slightly in the water, as they will be fully surrounded by ice - we like plastic insects and animals.  You could try marbles and stones, by freezing the tub in stages to make sure they don't all end up at the bottom.  Tip the frozen block out onto a tray and watch the children spend ages trying to get at the treasure!  You could provide plastic cutlery, or beakers of warmer water (but maybe this idea is better outside in the summer). Add glitter or food colouring to the water if you like.