What is the smallest and simplest seed for a random number generator?
Put a parallel resistor and capacitor between the A/D pin and ground.  Make the resistor fairly high, preferably well above the input signal impedance requirement for the A/D.  Make the RC time constant maybe around 10 &micro;s.  For example, 100 k&Omega; and 100 pF sounds like a good combination.

To get a value with some randomness, drive the pin high for a while, then set it to high impedance and take a A/D reading a few &micro;s later.  Particularly if you properly abuse the A/D acquisition time, the voltage it will see will be dependent on the R and C values, the pin leakage current, other nearby noise, and temperature.

Grab the low bit or the low two bits and repeat as necessary to get any number of random bits.

For a more random pattern, perform this procedure occasionally and inject the low bit of the A/D result into the random number generator you are already using.