I remember playing with crystals as a kid and being utterly fascinated by the slow process of watching them grow! Those were the days spent outdoors though; nowadays, parents would have to plan carefully if they want to replicate the experience indoors.
Anything to do with water is also another cheap, easy, and entertaining activity. A simple bucket of water and some cups or small containers can provide hours of fun for toddlers and young children. It's so soothing to watch them play without realizing they're learning about gravity and spillage! For older kids, you could introduce the concepts of density and volume.