Erawan National Park is the classic full-day trip from Kanchanaburi: a forested limestone park whose main trail follows a chain of seven turquoise waterfall tiers. The lower levels are easy to reach and popular with families; the upper pools require a steeper climb and feel progressively wilder.
The park is roughly an hour and a half northwest of town by road, so start early if you want unhurried swimming time before the tour buses fill the lower pools. Wear shoes that can handle wet rock, and keep food sealed; the fish and macaques around the pools are bold.