The opposite of a microwave, in terms of heating, would be an appliance that cools quickly. there isn't a common household appliance specifically designed to cool items quickly. The main reason for this is that cooling requires the removal of heat from an object, which is a more complex and energy-intensive process than heating.
In everyday household appliances, cooling is generally achieved by refrigerators or freezers. These appliances work by transferring heat from the interior of the unit to the exterior, using a compressor and refrigerant. While they can effectively cool items over time, they are not as quick as microwaves in terms of heating. The technology and mechanisms involved in cooling are different from those used for heating, which is why we don't have a readily available household appliance that can rapidly cool items.
There are, however, some specialized industrial or scientific cooling equipment that can cool certain items more rapidly, such as blast chillers used in commercial kitchens or liquid nitrogen-based cooling systems in laboratories. These systems are not typically found in regular households due to their complexity, cost, and specific use cases. For everyday household purposes, we rely on refrigerators and freezers to keep our food and beverages cool over a longer period of time.