If you insist on making Manhattan the site of your scenic ride, you're going to have to either take a subway from Penn Station to another station OR brave New York City traffic to get to one of the paths on the east or west sides of the island. The west side bikeway is much nicer. If you exit Penn Station on the 8th Avenue end, ask someone how to get to the West Side Highway. Strap your helmet on tight, keep your eyes and ears wide open and you should be able to make it to the path in one piece. I've done it many times. My wife is not such a seasoned rider, so I wouldn't even think of taking her through the city streets.

The Central Park loop is a great ride, but very hilly in some spots. You said you want to keep it simple.

If you live in South Jersey - and you're tired of out-of-the-way, boring "rail trail" type rides (Henry Hudson Bikeway, Edgar Felix Bikeway, etc) - why not try the new waterfront walkway from Hoboken to Liberty State Park? It must be finished or ALMOST finished by now. My wife and I rode the incomplete route twice last year and loved it. It was scenic enough for me (great views of the city and a gorgeous end at Liberty State Park) and easy enough for my wife. You can end with lunch or dinner on Washington Street in Hoboken and then grab a cannoli at Carlo's Bakery.

Bottom line: Unless you're really experienced and comfortable in city traffic, there is no simple way to reach a scenic ride in Manhattan by bicycle. For the West Side Greenway, take the #1 to the end of the line at South Street. Take a little detour through Battery Park, then work your way up to the greenway. Expect LOTS of pedestrian and bike traffic if you go on a weekend, though.

Enjoy. It's a great way to see New York!