You can also wire 2 in parallel with current limit resistor(s) so the current in/out of the pin is not exceeded.
I just completed a project where I had 2 RED LEDs in series on some pins and 2 BLUE LEDs in parallel on other pins.
For REDs I used a 100 ohm resistor, and for BLUEs a 200 ohm resistor.
These were the 5mm high brightness LEDs that dipmicro.com sells for 5-6 cents each.