Rockland County.
It's the most Jewish county in the US, 31 percent of the population is Jewish. You're only about 20 miles from Manhattan on the west side of the Tappan Zee Bridge. The Pascack Valley Line is run by NJ Transit, technically Metro-North holds three stops at Spring Valley, Nanuet and Pearl River. It'll take you to Penn Station in about 75-85 minutes.
Monsey has a large orthodox Jewish population, which tends to run a little exclusive, I'm not sure of your denomination. Airmont has a rising orthodox haredi population as well, though not as monolithic as Monsey. Basically, from the Garden State Parkway, draw a line northward along Rte 45. East of that line, there is a large reformed/non-Orthodox Jewish population. West of Rte 45, the Jewish population is largely Orthodox.
Several towns are small and walkable. Nyack, Pearl River, Nanuet, Stony Point, New City, Valley Cottage. It will really be up to your tastes. I suggest a day trip, or at least Google Maps streetview to see if anything looks appealing.
I suppose you have a lot of reading and research ahead of you. Hope these help you in your search:
A little about the Jewish population of Rockland County:
Rockland County
Jewish Federation of Rockland:
The Jewish Federation of Rockland County
Article in the Jewish Week about an organization recruiting Jewish families from NYC to Rockland County:
Rockland Outreach Facing Demographic, Image Challenges | The Jewish Week
Interesting radio program (WNYC - NPR, The Brian Lehrer Show), about how the Hasidic population are making cuts to the public schools in Ramapo (western Rockland County), though they do not send their children to the public schools.
The Brian Lehrer Show: Rockland County Conflict - WNYC
Blurb about Ramapo's growing Orthodox Community but also growth in Rockland County in general:
Rockland County, NY - Growth of Orthodox Jewish Population Leads to Home Building Boom -- VosIzNeias.com
Best of luck.