Serial monitor opening

Is there any way to open the serial monitor just by plug in an arduino board?

On an honest to god Uno yes, you can plug a serial device into pins 0/1, and use Serial.print, etc. I believe Liudr’s serial lcd keypad would work:

However, on Teensy’s, Leonardo based chips, Arm based chips, etc. you will need to change your code to use Serial0, Serial1, etc. instead of Serial to write to the device as opposed to the USB port. This is due to the Uno having a second AVR processor in it that manages the USB port and it can do both the pins and USB without changing code. The chips without a second processor can’t do that, and they need to know whether they are talking to a serial device or USB.

Besides a serial monitor, you could also use a bluetooth serial device that you plug into the serial ports (or software serial ports), and then use an app on your smart phone to read/write the bytes.