Sandy beaches that is...I tend to find very little to do on beaches (as I am in need of constant stimulation) so I don't have much to say for each except they are all unique.
So to make it simple, here are some of my favorites:

Hampton Beach State Park, Hampton, NH
Odiorne Park, Rye, NH
Jenness State Beach, Rye, NH
Long Sands Beach, York, ME
Fort Foster, Kittery Point, ME
Great Island Common, Newcastle, NH
North Hampton State Beach, North Hampton, NH
Seapoint Beach, Kittery, ME
Wallis Sands State Beach, Rye, NH
Parker River Wildlife Refuge (seasonal) Plum Island, MA
Caps Cod National Seashore, Cape Cod, MA
Reid State Park, Georgetown ME
Goose Rocks, Kennebunkport, ME
Ogunquit Beach (and the Marginal Way) Ogunquit, ME
