The Halden Canal was built in the 19th century to float timber down from the forested lake country behind the town, and the lock system at Brekke — a set of chambers that step boats up between two lakes — is the most visited stretch of it. The locks still operate for pleasure boats and the small canal boats that run the route in summer.
It's a pleasant half-day trip out of town: forest, water, and a working piece of 19th-century engineering, quieter and greener than anything in Halden itself.