If "kalpana" means imagination:
* वास्तविकता (vaastavikta): Reality
* सत्य (satya): Truth
* अनुभव (anubhav): Experience
If "kalpana" means a plan or idea:
* कार्य (kaarya): Action
* क्रिया (kriya): Action
* निष्पादन (nishpadan): Execution
If "kalpana" means fantasy:
* वास्तविक (vaastavik): Realistic
* प्रायोगिक (prayogik): Practical
* यथार्थवादी (yatharthaavadi): Realistic
The best opposite for "kalpana" will depend on the specific meaning you are trying to convey.