We've had this set for about a year and I play it with Zeah, she plays with it alone, and we take it when I babysit some other homeschooled children. It's been really popular with 3-8 year olds. We have the version pictured, with activity cards.
Zeah uses the pebbles for sorting (size, colour, shape), stacking, counting, and making patterns. The pebbles are ideal for loose parts play, as maths manipulatives and the activity cards prompt plenty of variations of learning and play. There are several sets available.