A valuable item (adjust the price in gold pieces to whatever appropriately entices your players) is trapped in a block of magical ice.
The ice melts as normal if heated, but the loss of weight triggers the release of a suspended pendulum (a sickle) made of magical ice. This swings down at any creature standing immediately before the ice block, and deals 1d12 (half slashing, half cold) damage.
Snowflake by Aaron Burden, modified by Seth in GIMP on Fedora Linux. Unsplash License.