Sentences that use synecdoche:
After he won the lottery, he bought the most expensive set of wheels on the market. (Here, “set of wheels” refers to an expensive car.)
The customer asked the salesperson if the store accepted plastic or just cash. (Here, “plastic” refers to credit cards.)
The pair of eyes were watching her every move. (Here, “pair of eyes” refers to a person.)
Sentences that use metonymy:
The White House issued a press statement. (Here, the “White House” refers to the US president and government officials.)
The restaurant has become increasingly slow and rude. (Here, the “restaurant” refers to the staff that works there.)
That is his go-to dish—he makes it whenever he has people over. (Here, “dish” refers to a type of food.)