Extending a tourist visa in Sri Lanka - is it straightforward?

You can extend at the Department of Immigration and Emigration in Colombo. It's generally straightforward for tourist extensions up to 6 months total, but requires paperwork, photos, and a fee. Processing can take a few days to a week. Many people do it successfully. Start the process before your current permission expires. Check the official immigration website for current requirements and fees as they can change.