Go to New York in the early spring or early fall. These are the times of year that are perfect for sightseeing. It's far enough away from Christmas and the kids are still in school so the crowds won't be as bad throughout New York.
Wake up early and get out as soon as you can. Just as with most major cities, New York isn't as crowded early in the morning. Sightseeing before noon is always a good idea while in New York.
Schedules personal tours at different destinations in New York City. Chances are that you'll still be with a group during these sightseeing tours but the crowd won't be anything that you can't handle.
Take a bus tour throughout Manhattan. These sightseeing tours will actually keep you away from the crowds since you'll be on a bus for a few hours. You'll see plenty of the tourist hot spots without having to fight the crowds.
Consider taking a ferry tour around the island. The Circle Line tour, for example, is a nice way to spend the day and allows you to see the city from a different perspective. Much like the bus tours, this keeps you far away from any and all crowds found in New York.
