Comment Light bulbs and batteries (Score 1) 364
Firstly, don't knock light bulbs and batteries as they teach the basic concepts of series, parallel, and series-parallel circuits and digital logic (AND, OR, NOR, etc), secondly for $5 dollars you won't be doing much with modern electronics as they require FPGAs, ASICs, DSPs, etc. A silicon diode has a 1V/degree celsius response and as such makes a handy temperature gauge. I have used this as a college lab where students take a temperature reading, apply heat and then take a reading with the diode thermometer and calculate the change in temperature. This helps the students understand the role of components as well as the effect of temperature on electronic components.