In The Great Gatsby what does Myrtle Wilson buy while she is out with Tom and Nick?
Myrtle gets a dog when she and the men reach the city. Her choosing such an item demonstrates to Nick--an acute observer--that Myrtle and Tom's relationship is and has been ongoing. They have an apartment in the city, and dog ownership suggests that they plan on continuing their relationship indefinitely.
This scene is also important to the clue that George (Myrtle's husband) discovers later in the novel, proving to him that his wife is having an affair. I don't know how far you've read; so I don't want to give too much away, but just keep the purchase of the dog in mind as an example of foreshadowing.