Buying Ready-To-Use DIY Simple Electronic Project

I need a straightforward, fully-functional DIY project that I can assemble and demonstrate without extra troubleshooting. Any of the three domains—electronics, electrical, or instrumentation—are acceptable,... (Budget: $10 - $30 USD, Jobs: Arduino, Circuit Design, Electronics, Embedded Systems, Instrumentation, Microcontroller, PCB Layout, Raspberry Pi)