Hi Cannelle,
If you love culture, I’d suggest visiting the north first:
there are the Dambulla caves,
then the city of Polonnaruwa,
and of course Kandy.
The journey from Colombo is really slow (8 hours) for just 1-2 €!!!
The best thing is to stop in Polonnaruwa for a few days (check out my video below),
then take a taxi to Dambulla (70 km and 1.5 hours) and return.
As for Kandy, Colombo-Kandy—yes, stay a few days, then take the bus (easy) to Dambulla and Polonnaruwa.
For the bus, it’s dusty => bring a scarf and cap to protect yourself, and earplugs or noise-canceling headphones, because the music is LOUD!!!
As for the south, I’d save it for last—take the bus (comfortable here) or train to Galle for a stroll. The temperature is very humid (coastal) and stifling.
Polonnaruwa:
https://www.youtube.com/watch?v=Yeqs5IBEpj8&list=PLY-bryruKxP8DjiLPCgYQPw623RJm84cs&index=46Dambulla:
https://www.youtube.com/watch?v=J0V6po1EOeg&list=PLY-bryruKxP8DjiLPCgYQPw623RJm84cs&index=53