Cold Spring Harbor Fish Hatchery and Aquarium
1660 Rte. 25A
516-672-6768
Open daily 10am–5pm
Adults $6, children $4, under 2 free
The hatchery has both indoor and outdoor exhibits showcasing the diversity of wildlife in New York State's waterways; including fish, reptiles, and amphibians. Catch-and-release fishing is also available; check their website for more information.

St. John's Pond Preserve
Rte. 25A, just behind the Fish Hatchery
Open daily 10am–5pm
Free
Visit some of Long Island's oldest forests as you climb a steep hill and descend to spectacular views of St. John's Pond. Although extremely rare, river otter were spotted here in 2008. To access the trail, you must pick up a key to the gate at the fish hatchery. 

Cold Spring Harbor State Park
Harbor Rd., adjacent to the library
This park offers 40 acres of hilly terrain. Those who can manage the steep climb will be rewarded with the panoramic view of the harbor. You can access the Nassau-Suffolk Greenbelt Trail here and hike to Bethpage State Park or Montauk Highway in Massapequa, 26 miles away.

Uplands Farm Sanctuary
Lawrence HIll Rd.
Open daily dawn to dusk
Of the sites listed here, this is the best suited for young children, as the 2.4 mile loop is relatively flat. If you prefer a longer hike, you can access the 26 mile Nassau-Suffolk Greenbelt Trail here as well. In the meadow, you can spot a variety of birds and butterflies. Although no longer a working farm, the farmhouse and silo still exist and now house the Nature Conservancy offices.