Unproject, learning

So I am stuck in town without much to do. I got another arduino board under the tree and have a USB cable handy, but not much else.

Obviously the power of arduino is in connections, but has anyone got a decent noob-to-intermediate project or exercise for a plain board other than making it blink?!

you can make a calculator, that blinks the led as often as the answer?

you can make a prime number generator...

And you could use the serial monitor to send characters.... so you can give the board some input that way

Edit: See for instance here as a start.