Grab breakfast or a cup of coffee at one of the Ground Central locations nearby. Once you've checked out of the hotel, walk to Vanderbilt Market (10 minutes) for lunch. This market has over 15 varying food stalls with lunch and drinks options. If you want an NYC shopping day, walk 10 minutes to Bryant Park and check out the New York Public Library (see if they have a cool event or exhibit!) and park before walking north or south on 5th Avenue for large-scale shopping. If you want a Central Park Day, walk 30 minutes, or take the 6 train 1-stop north to Lexington Avenue/63rd street and walk east for a few blocks until you see the park. You can rent bikes, walk around, and enjoy the fun and beauty that is Central Park!
Grab a hangover burrito at the Santa Fe counter, then walk 5 minutes to McCarren Park to enjoy it while dog- and people-watching. Another 10 minutes east will take you to the waterfront Marsha P. Johnson State Park, where you can enjoy a classic view of the Manhattan skyline. If your hangover is cured by some hair of the dog, grab a beer and greasy burger from Spritzenhaus or Radegast beer halls. Bathhouse Williamsburg is a great place to have a relaxing spa day, though you'll likely need to book a reservation in advance.