For anything past the closer waterfalls — or when friends fly in and it's suddenly five people, not one — I switch to a van with a driver.
Same idea as the tuk-tuk: driver included, no self-drive, and someone who already knows which roads flood.
Air-conditioned, enough boot space for actual luggage (not just a day bag), and comfortable enough for the longer haul out toward Nuwara Eliya or the Knuckles range.
Costs more than the tuk-tuk per day, obviously — worth it once you're splitting it four or five ways.