Soon she'll realise that more water makes it sticky, less makes it too hard to handle, and drying out changes it into a solid. A solid that is not solid and a liquid-like substance that is not liquid. Again, scale matters more than quantity. Failing sand, a kilo of rice is sometimes a worthwhile gift. But she is ready to observe that shiny stones dull as they dry, green twigs bend but later snap, and the world is full of fascinating shapes, textures and creatures.
Different colours are fun, but different shapes are more important.
They must all be in scale so that tiny ones are quarters and small ones are halves. There is scope here for making as well as buying. Plastic beakers that will build up or fit inside each other as well as being used in the bath are a cheap and easy version of many stacking toys. She will probably also like a first jigsaw where whole figures lift out by a knob. Soon she'll be ready to tackle standard jigsaws with a few large pieces.