We had a great time, collecting rocks, playing catch and taking videos.
The Beach Rock Mat
Smooth Rocks, about the same size.
Rubber Door Mat, I used this.
Frolic on the beach and gather rocks, lots of them. On the backside of the mat, layout all the rocks so that they fit nicely and are space the way you like. Starting at a corner, apply glue to the bottom of the rock, secure to mat. Make sure to have a window open, unless you want to get high while you craft. Continue till all rocks are glued to mat. Let dry and enjoy!