Are there any substances that react very slowly with water to create heat?
Hand- and foot-warmers are made containing iron filings, with a bit of paper pulp and salt, and sometimes a sealed water packet. These rely on the (surprisingly rapid) rusting of iron in contact with air, catalyzed by the water and salt (the pulp is just to hold the water and keep it in contact with iron). This reaction is a fairly safe way to melt a bit of snow, with some caveats:


It's more expensive than using freezing point depression of salts such as $\ce{CaCl2}$, $\ce{Mg(C2H3O2)2}$, or $\ce{NaCl}$ (table salt). Even adulterated wine containing ethylene glycol has been used for runway deicing.
The rust will eventually leave brown streaks on any surface below the deicer, such as the walls of a building if used to melt snow on the roof.