In your DAC, the output resistance of the R-2R ladder is R. You can connect a resistor from the output to GND to attenuate the signal. A resistor of value R will, in your example, reduce the output swing to be zero to 2.5V.
Keep in mind that, if you want the output to be monotonic, you should use 0.1% resistors for an 8 bit DAC, because the LSB is 0.4% of full scale. Also, the output resistance of each logic stage adds to the value of R.
I might use 1Meg and 499k, 0.1%. You can also buy R-2R networks with guaranteed +/- 1/2 LSB accuracy.
I realize that you don't always need this kind of accuracy, but I think it is good to know, so you don't get any surprises.