Scratch for ComputingScratch is a superb programming environment for the teaching of computing (computer science) and computer control. A good starting point is the Scratch chapter form The Raspberry Pi Education Manual.
Control programs can be simulated/modelled on screen using sprites (see the Lighthouse v2.sb, level_crossing_simulation v2.sb and Computer Controlled Vehicle.sb examples below). With the addition of a Scratch compatible interface it is possible to incorporate an element of physical computing.
PicoBoard - these can be purchased for about £40. They are and input only device. The board has a number of built in sensors for light and sound, it has four inputs for resistance measurement, a slider to give a variable input and a push to make switch input. It can be used with cheap, off the peg, components such as LDRs, and thermistors (see Data Logging page on this site and Pico barrier.sb example below).
LEGO WeDo - a WeDo kit can be purchased for about £110. The WeDo interface has the advantage of being an input/output device but it functions are very limited. It comes with two inputs - a tilt sensor and a distance sensor. The output is to a motor. (see WeDo Alligator v2.sb, WeDo_flight_simulator.sb and WeDo_vehicle distance sensor v2.sb examples below).