Probably not the best way, but I take the Brooklyn Bridge, then go across town on Chambers and up the Manhattan Greenway. It is a pain to dodge tourists on the Brooklyn Bridge but I try to leave enough time to enjoy the view (having a bell helps). To get to the bridge from Park Slope, I take 4th or 3rd Avenue to Bergen Street, Smith Street, then either Jay Street to Tillary to the bridge, or Schermerhorn to Boerum Place to the bridge. The Manhattan Bridge is much more peaceful (no, or very few pedestrians!) however I use it primarily when my destination is on the East Side. Are you familiar with Ride the City, where you can map point-to-point routes? Google has a similar route planner, I believe.