Here is an idea, may not be trivial, but would be useful for my lab bench.
A PICAXE power source! Use PWM to run a FET, that chops to create a 40V, 2A max DC output, filtered to a certain extent. It could be open loop to start.
That means there is no feedback, as in using the ADC input to compare the actual output to the commanded one, and using negative feedback to correct.
It would be nice if adjustable in 0.1V increments, and be settable thru some kind of PC interface, plus readback the actual voltage on the screen. This would exercise a variety of software and hardware, and actually help in making circuits. For extra credit, add a current limit/constant current mode, for safety and charging some batteries, like lead acid, nicad, and NIMH.