Sponge blocks. You could also cut different shapes for a soft, colorful block set. You could even use these in the tub. Cheap Activities For Kids, Sponge Block, Toddlers Approved, Indoor Recess, Fine Motor, Quiet Activities For Kids, Sponge Towers, Kids Indoor Activities, Wooden Block
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Finger painting using sponges-brilliant!
Teach colors to preschoolers. Lots of fun preschool learning activities