Here is a simple, free activity that can be done practically anywhere (as long as there is grass) and will keep the kids amused for a while, it is something that I did when I was younger and that I have done with Thomas.
What you need to do
Select your blade of grass; thin flat ones work the best, you may want to give it a wash first and make sure that the grass hasn’t been sprayed.
Pinch the top of the blade of grass between your thumb and first finger and bring the bottom of the grass down to the base of your thumb.
Put both of your hands together so that your thumbs and their bases are together trapping the piece of grass.
Make sure that the blade of grass is taut.
Purse your lips together and blow between your thumbs and over the blade of grass.
You should hear a high pitched whistle noise.
It may take some practice to get a ‘whistle’, try making the grass more or less taut and have a go with different types of grass.