There are several synonyms for the phrase "plopped down," all of which describe a sudden and heavy settling into a chair or other surface. One alternative is "flopped down," which suggests a lack of energy or enthusiasm in the action. "Collapsed into" can also describe the suddenness and weight of the movement, while "slumped into" implies a more gradual sinking into the surface. "Dropped into" suggests a deliberate but sudden movement, while "plonked down" connotes a more careless or abrupt settling. Other options might include "sank into," "settled heavily," or "dropped with a thud".