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Play dough is a classic, open-ended sensory activity. You can go with the classic Play-Doh brand (I bet you can get a few colors for just $1-2 at the Dollar store) and get lots of awesome accessories to encourage creativity and keep this fun for a long time.
You can also get an eco-friendly play dough with all natural ingredients, an allergy-friendly play dough that is free of wheat, soy, and other common allergens. This one also has an extra sensory bonus because it is aromatic.