You can't visit a new city without coming home with a souvenir, so make sure everyone gets a creative favor from your trip to the city. Customize your favors to say "Abby's City" or "I had a blast at Noah's City Bar Mitzvah" your guests will love this creative addition to the party.
The birthday guy or gal is unique. Their favors should be unique too. Don't settle for a standard, run-of-the-mill favor to give out at your city, personalize something so it has extra sentimental value. Personalization means that guests will cherish their favors, not just throw them out before the Bar or Bat Mitzvah Celebration is over.