Serial control of 25 LEDs from a single pin
Shift Register and I2C is good, but only if there are at least 2 pins free. I suggest Dallas 1-wire port extender DS2408 will be an optimal solution. Code for Dallas 1-wire device access is not so complicated and there are plenty of examles on the web.

You may use several DS2408 connected to one pin or implement more logic after single DS2408 whichever you'd like.