Søndermarken is the park south of Frederiksberg Have — twenty-six hectares, more wooded, less formal, designed in the 18th century as the rougher half of the royal grounds. The contemporary draw is Cisternerne: the old underground water reservoir Copenhagen built in 1856 and decommissioned in 1933, reopened in 1996 as an art space. The glass pyramid on the lawn is the only above-ground sign.
Three kilometres, ninety minutes including the underground exhibition. Cross from Frederiksberg Have through the side gate (or come direct via Frederiksberg metro and walk south). Søndermarken's main paths run through old planted woodland — fewer manicured lawns than its sibling, more space for the trees to settle. A pond, a 19th-c orangery, a couple of statues.
Cisternerne's entry is the glass pyramid west of centre. Descend the staircase; the temperature drops ten degrees. The exhibition is whatever's installed — usually a single artist taking on the reservoir's two-metre-deep water and brick vaulting (Hiroshi Sugimoto, Jeppe Hein, Tomás Saraceno among recent shows). A different walk inside.
Back up via the wooded paths to Roskildevej and the metro.