This is the customary blinking LED project that every Arduino newbie starts with.

Plug a LED Brick into Port 2A of the Microcontroller Brick and run the program:

var led = new Led(PORT_2A);
while(true) {
  led.setState(true); delay(1000);
  led.setState(false); delay(1000);
}

The LED should blink at 1-second interval:

Plug another LED Brick into Port 2B and modify the program as follows: 

var led1 = new Led(PORT_2A);
var led2 = new Led(PORT_2B);
while(true) {
  led1.setState(true); led2.setState(false); delay(1000);
  led1.setState(false); led2.setState(true); delay(1000);
}

Now you have two LEDs that blink in turn at 1-second interval.

How about throwing a Pushbutton Brick into the mix?

Connect a Pushbutton Brick to Port 3A. Now modify the program as follows:

var led1 = new Led(PORT_2A);
var led2 = new Led(PORT_2B);
var pb = new Pushbutton(PORT_3A);
while(true) {
  led1.setState(true); led2.setState(false);
  while(!pb.clicked());
  led1.setState(false); led2.setState(true);
  while(!pb.clicked());
}

Now the LEDs take turns lighting up when you click the button.