This combo lock is a great tool for teaching three-digit numbers, i.e. 1's, 10's and 100's.
I taught the kids that the first number is the hundreds, the second is 10's and the last one is 1's. I can set the lock to any number, say 1-3-7, then hand it to them and call out "one-hundred and thirty-seven." Sometimes, they get it right away. Other times they work on it together. Occasionally, they get stumped when I trick them, like "four" which is 0-0-4.
I can carry this in my pocket, my pack or just my hand and it takes less than ten seconds to change the code. We can walk in the forest (or anywhere), taking turns opening the lock and mastering our understanding of numbers 0-999. The best part is that satisfying little click when you know you got it right.
No, wait. The best part is that it produces no garbage and we can reuse it over and over. No, the best part will be when I start giving them addition and subtraction problems. No, here's the real best part - the kids love it.
I bought this thing for six bucks. You probably already have one lying around your house.