This is by no means a new product, but it's fun and worth mentioning. B.lab, an Italian manufacturer, has a product called B-surfaces, it's a floor tile that has a layer of liquid beneath a clear coating. When pressure is applied to the surface of the tile, the liquid moves to an area with less pressure, leaving a negative impression of whatever applied the force for a few moments. (If you walk on the tile, your footprints will remain for a few seconds.)
We had this installed in a study area at my college and it was definitely a fun interactive experience. This tile would be great for any place where children will be present. B-surfaces is sold under many different brand names: Liquifloor, Liquid Floor, Liquid Lava & Liquid Fusion, to name a few. Tiles are available in a variety of colors and in Watercolor (1 color), Bi-color, or Metal. Tables with B-surface tops are also available.