Ulysses S. Grant is buried (interred in an above ground sarcophagus) at General Grant National Memorial, informally known as Grant's Tomb in New York, New York.
Location: General Grant National Memorial, Riverside Park
Address: Riverside Drive and West 122nd St, New York, New York
As seen in the photograph above, Grant is interred in an above ground sarcophagus. Therefore, the answer to the question, "Who's buried in Grant's tomb" is correctly answered, "no one."
For photographs of the graves of all other U.S. Presidents visit the Presidents Gravesites page.