This Arduino project for beginners uses 24 LEDs to make a Knight Rider display. The LEDs are connected to the Arduino pins using series resistors.
Find the circuit diagram and Arduino code at Starting Electronics website.
An op-amp is used as an oscillator in this LED flasher circuit. This is another circuit that can easily be built on an electronic breadboard by beginners in electronics. Click the above link for the circuit diagram.
This circuit uses a 555 timer IC and produces a fixed frequency audible tone that is played through a loudspeaker. No amplifier is necessary as the 555 puts out a signal of high enough amplitude. The circuit is easy to build for beginners in electronics and can be built on an electronic breadboard. Click the above link to find the circuit diagram.
A buzzer can be directly connected to an Arduino if it operates at the same voltage as the Arduino (usually 5V) and does not need to draw more current than an Arduino pin can deliver.
But what if the buzzer operates at a different voltage to the Arduino and / or needs to draw more current than the Arduino pin is capable of supplying? One solution is to use a transistor to interface the buzzer to the Arduino. The Arduino buzzer transistor connection circuit on the Starting Electronics website shows how to connect the buzzer to the Arduino using an NPN transistor.
Note that the piezo buzzer used in the circuit is not a plain piezo element, but sold as a piezo buzzer that has built-in circuitry that makes the buzzing sound when power is applied to it.