Take them to char dham yatra.
Lots of people thought char dham is for old age trip only but it is totally a misconception.
These places are having beautifull landscapes and a complete package for all age group travellers.
Hope you best of luck.
Once my parents take me when i was young and once i take them to Dwarkadhish and Jagganath Puri.
Yes I did.
on my 22nd birthday I took my parents to a South india trip of 10 days.
they loved it.
the best part was it was the first time they board a flight.
you should also they your parents they'll love it.