It really depends on his and your interests. Viator is a fab site for good ideas. Carriage ride in Central Park etc. If he's into movies then maybe, for example a Ghostbusters DVD and promise to visit the fire station. If he likes music, then are there any American bands he likes with links to New York City? Lunch at Empire State? Lunch at Grand Central station? Comedy store, it's a different prospect to those in London. Tickets to ice skating in Central Park. Tickets to the Radio City Xmas Spectacular is an institution there. Tickets to to shows like Jerry springer. Phaidon do good guide books that are cooler and more bespoke than some. Urbandaddy.com is a good local site for ideas.