If you have one or more 0.1% reference capacitor(s), you might be able to measure resistance by measuring RC time constant.... You charge and discharge the capacitor via one digital output through the unknown resistor. The voltage at the R-C connection is measured with an analog pin and the output inverted when the cap has charged or discharged a given amount (relaxation oscillator).
So long as the time constant is fairly long you can get good accuracy (you only sample the voltage every 0.1ms or so with the ADC, so a time constant of 0.1s or more is needed). Take 1/4 and 3/4 of the supply as endpoints, thats analog values 256 and 768, and you'll be switching outputs every 1.0986 x R x C seconds. ln (3) = 1.0986
You'll have to allow for the output resistance of an output pin though. And the fact that capacitors aren't as ideal as you might hope.