Your are going to be walking everywhere, because all the cute little shops are practically on top of one another. You can walk across the bridge from Lambertville to New Hope and back, just be sure to wear shoes that are not too wobbly, because the walkway consists of uneven wood planks!
We drive and take Route 31 or Route 29 for the most scenic views of the Delaware and the NJ countryside. There is also bus service from NYC on Trans Bridge Lines. Call them at 1-800-962-9135 for more information.
Walk. Park the car and walk. Good luck with parking the car. Metered parking means that you might be making multiple trips to feed the meter. They will ticket if you have an expired meter.