As mentioned in both contributions above, filtering the squarewave with a lowpass (or bandpass) is the most direct way to produce "something like a sinewave". The required filter order depends on the desired quality of the sine wave (THD).
However, this method works only for a fixed frequency.
Another - a bit unknown - method uses the classical differential amp (longtailed pair) to convert triangle waves to sine waves. This is in a broad frequency band independent on frequency, however, this method works only for a fixed amplitude.
Example: for an amplitude of 80 mV you get a sine wave with a THD of app. 1.5%.
Disadvantage: The squarewave has to be converted to triangle before (Integration)