147 acres sounded like an easy morning. It wasn't — the Royal Botanic Gardens at Peradeniya swallowed an entire afternoon and I still skipped whole sections.
Giant bamboo, a canopy walk, and an avenue of palms planted by actual royalty. Wear shoes you can walk in for hours.
The orchid house (see that write-up separately) is inside the same grounds, so pair the two if you're only making the trip once.
There's a small entry fee at the gate — cash is easiest — and a few food stalls near the river entrance if you need a break.