There's a Circle of Life Rediscovery blog post about outdoor maths: "6 ideas for using syringes in a mathematical way outside" written by a respected outdoor educator. It was featured as an NAEE blog last week.
"I’ve always used syringes for water play, mark making, as air pumps in technology projects and for having fun in the snow. I’ve always chosen the biggest syringes I could find – 100ml ones. But this set, a present from a friend, fuelled the mathematical fire within me. Have a close look at the sizes and see what you notice – this is just the sort of thing to ask older primary aged children."