Rhode Island, by the way, is another great place to go a'letterboxin':

Trustom Pond

A delightful and gentle two-mile walk on the Rhode Island shore with
views of salt ponds and barrier islands, and splendid birding, for a
pair of letterboxes. You could also easily include the "Trustom Pond
Quest" letterbox along the way for a triple.

Directions: In Charlestown RI on Rte. 1 between Moonstone and
Ninigret Beaches, turn south to Green Hill Beach. This is Post Rd:
after a couple hundred yards, turn left onto Green Hill Beach Rd and
follow it for 0.7 miles to the end. Turn left at the stop sign, then
quickly left again onto Matunuck School House Rd. After 0.8 miles,
find the Trustom Pond National Wildlife Refuge entrance on your right.

The Trustom Pond NWR occupies old farmland bordering salt pond
estuary. There are two connected loop trails, each with a southward-
pointing extension that leads to observation towers over the
wetlands. From the parking lot, if the small headquarters is open,
grab a map. Then follow the well maintained path past a map board to
skirt a field and pass a farm pond. The trail turns to grass. Turn to
the path bearing 144 degrees, and then at another map, go left to 134
degrees. Follow this straight old farm road past a lovely bench and
out to the observation tower: the East Point Letterbox is behind the
stairs, under a flat rock.

Now return to the map by the farm pond and turn onto 244 to cross a
short boardwalk. At a wide intersection, go 270 degrees and then 282
degrees at the next intersection, towards the second observation
tower. This path will lead you up to another crossing with a bench
and map: turn to 146 degrees. This larger, more isolated observation
tower is the place for a rest! Again, peak behind the stairs and
under a flat rock for the West Point Letterbox.

When you've had your fill of this expansive place, return to the last
map to plot your next adventure!