Fluidity is a wonderful free sensory app. When you swipe your fingers across the screen you move the fluid into an array of colours. This app is a bit like a lava lamp at your fingertips. Children love it. It works on both iPad and iPod.
Pocket Pond is a really popular free app in our school. It is basically an interactive fish pond. When you touch the screen you can hear the water splashing and fish swim off. You can add lilypads and noisy dragonflies. It works on the iPod and the iPad. Children love it.