Straddling Rockland and Orange Counties is Harriman State Park (off Rte. 17 or Palisades Pkwy., Exits 17 or 18, 845/786-2701, 8:30 a.m.–dusk daily, parking $6–8 June–Sept. and weekends year-round), a 46,000-acre preserve that is considerably less developed than its better-known neighbor to the north, Bear Mountain State Park [1].
Through the heart of Harriman runs Seven Lakes Drive, which hugs the shores of only a small portion of the many bodies of water to be found in this preserve. Two of the most spectacular of these are crystal-clear Lake Tiorati and Lake Sebago. Ironically, their Native American names—bestowed upon them by white men eager to create a romantic atmosphere—are not local, but rather names that come from Western tribes. Both of the large, artificially constructed lakes have swimming beaches.
About 200 miles of marked trails loop through Harriman and its neighbor, Bear Mountain. Basic information on some of these trails can be picked up at the headquarters of the Palisades Interstate Park Commission (Bear Mt., Rte. 9W, 845/786-2701). For more detailed information and maps, stop at the superb Park Visitor Information Center (between Exits 16 and 17 on the Palisades Pkwy., 845/786-5003) or contact the New York-New Jersey Trail Conference (201/512-9348, www.nynjtc.com [2]). The park also offers an excellent, 200-site campground; call 800/456-CAMP for reservations.
Just south of the park, on a mountaintop near Hillburn, is 
Mount Fuji (Rte. 17, 845/357-4270, $19–37), a big, glitzy Japanese steakhouse offering stunning views of the valley below. On the western border of the park are the Clove Furnace Historic Site, Tuxedo Park, and Sterling Forest [3].
Links:
[1] http://www.moon.com/destinations/new-york-state/the-hudson-valley/lower-hudson-valley/the-ramapo-mountains/bear-mountain-state-park
[2] http://www.nynjtc.com
[3] http://www.moon.com/destinations/new-york-state/the-hudson-valley/lower-hudson-valley/the-ramapo-mountains/route-17-south