Well, I discovered this great idea recently... make ice decorations!
It's so easy to do.
Simply choose a cheap container and fill it with water. Then get your little ones to pick up leaves, twigs, rosehips (with your help) etc... and put in the water. Then leave outside and let the cold weather do the rest. By the next day (frost depending) the water will have frozen. Then tap out the ice and put on display in the garden - you can create a hole by piercing the decorations with a hot knitting needle and threading with twine, string, ribbon - you choose!
I have to say I'm really pleased with the result and it fascinated my children too!