When you first start programming with an Arduino or other development board, the “Hello World!” of that genera is to blink the onboard LED. You probably want to go further though, which can be difficult if you don’t have a drawer full of spare parts.
You could purchase some sort of starter kit, or on the other end of the spectrum, you can always just buy parts from various online outlets at extremely cheap prices with very slow shipping. Though taking weeks or more to arrive won’t satisfy your need for instant gratification, at least you’ll eventually build up quite an electronics collection. If you can figure out what to order.
Another option is the S4A board. This device fits on an Arduino Uno, leaving several I/O pins open, and includes buttons, potentiometers, LEDs, a buzzer, and accommodations for two servos. These components are already attached, so no soldering or breadboarding is involved, you simply plug it in and use as a test device or teaching tool.
On the other hand, if you’d test your I/O with only a bunch of LEDs and a breadboard, that can be fun as well: