General antonyms:
* Realistic
* Pragmatic
* Practical
* Down-to-earth
* Sensible
* Rational
* Logical
* Cautious
* Cynical
* Skeptical
More specific antonyms:
* Worldly (meaning experienced and knowledgeable about the real world)
* Pragmatic (meaning focused on practical results rather than ideals)
* Cynical (meaning believing that people are motivated by selfishness)
* Skeptical (meaning doubting the truth of something)
The best antonym for "quixotic" will depend on the specific context in which you are using the word.