Hi Charlie,
Yep!  Cape Cod is my stomping grounds.  The great thing about Cape Cod, as I mentioned in a post some months ago, is that the north shore and south shore of the cape, though only 5 to 8 miles apart, differ markedly in marine fauna.  This is due to the fact that the south shore, on Nantucket Sound and Buzzards Bay, is warmed by the Gulf Stream, and the water here is often 20 or more degrees warmer than in Cape Cod Bay, only a few miles north.  So you can collect on shallow, grassy tidal flats with water temperature near 70 degrees F, then drive 15 or 20 minutes to a surf-pounded shore of boulders and rockweed, with water temperature around 50.
Cape Cod is shaped like an arm, held straight out from the shoulder, with the elbow bent upwards about 90 degrees, and the fist curled inward, just the way you would hold your arm to show off your biceps.  Most of the good collecting, especially on the south shore, is on the "lower cape" and "mid-cape" regions, that is, between the mainland and the "elbow" of the arm.  Once you round the elbow and head north toward Provincetown, onto the "upper cape", you encounter several miles of clean sand beaches, relatively devoid of life.  Very few rocks, very little algal growth, very clean water, and relatively few animals.  It's hard to find a shell on the beach up there, except perhaps an occasional surf clam (Spisula solidissima).  Nice swimming though.
On the south shore of the cape there are many good collecting areas.  A few that I like are:
* Falmouth - this is at the southernmost point of the cape.  There is a good mix of habitats here - some rocky areas, some sandy beaches, some muddy tidal inlets.  One spot I like is a sandy tidal inlet beside the Mara Vista Motel (I'm not sure if the motel still exists under that name - the danger of using manmade landmarks).  Here you can find Busycon canaliculatum, and sometimes Polinices duplicatus.  Even though there are no rocks, Crepidula fornicata can be found in the channel, attached to each other in stacks.  Sifting the sand provides unlimited numbers of Gemma gemma, with far smaller numbers of Lyonsia hyalina, Tellina agilis, Pandora gouldiana, Haminoea solitaria, and Acteon canaliculata.  The pilings under the bridge have Anachis avara, Anachis translirata, and some Urosalpinx cinerea and Eupleura caudata.
The rockier areas around Falmouth usually have Ascophyllum rockweed, and vigorously shaking a handful of it into a bucket will usually produce Bittium alternatum, Lacuna vincta, Littorina saxatilis, Mitrella lunata, and others.  And of course, the rocks have Littorina littorea and Mytilus edulis, as every rock in New England does.  Purple sea urchins (Arbacia) are also here.
Right next door to Falmouth is the village of Woods Hole, which includes the world-famous oceanographic institute.
* Cotuit, Hyannis, Yarmouth - these areas are relatively rock-free, but offer both open sandy beaches and sheltered grassy coves with silty sand or muddy sand bottoms.  The shallow, warm coves are more productive then the open beaches.  At most such locations you can find Busycon canaliculatum (and sometimes Busycon carica), Polinices duplicatus, Nassarius obsoletus, Nassarius trivittatus, sometimes Nassarius vibex, Anachis translirata, Anachis avara, Mitrella lunata, Crepidula plana (inside almost any large dead gastropod shell you find), Mercenaria mercenaria, Mya arenaria, Pecten irradians, Crassostrea virginica, Anadara ovalis, Anadara transversa, Laevicardium mortoni, Nucula proxima, Tagelus plebeius, Ensis directus, Tellina agilis, etc.  Also pipefish, spider crabs, horseshoe crabs.  If the shore is muddy and grassy, you will also find Geukensia demissa in large numbers.
* Chatham - this is on the elbow of the cape, and here you are getting into uninterrupted sand.  Monomoy Island was a pristine spot where you could walk a mile of beach and only see a few other people.  Notice, I say "was".  I went there last summer, after a couple of years absence, and found they had built a public bathing beach.  However, down the shore from the bathing beach (to the right, as you face the water), there are several shallow, sandy coves and inlets in which surf clams and a few other sand-loving bivalves can be found, along with Polinices duplicatus and sometimes a few Busycon.  Also here is the beginning of the sand dunes which characterize the upper cape.
A few of my favorite south shore sites are not actually on Cape Cod, but a short distance away on the mainland.  A couple of these are:
* Harbor Beach, in Marion - a warm, shallow bay with a slightly muddy sand bottom, and lots of grass.  A bit hard to find if you don't know your way around.  You can walk or snorkle over 100 yards out and still be in 5 feet of water or less.  Nassarius (Ilyanassa) obsoletus and Mya arenaria are abundant on the beach area.  Bivalves include Laevicardium mortoni, Geukensia demissa, Crassostrea virginica, Tellina agilis, Ensis directus, Tagelus plebeius, Anadara ovalis, Anadara transversa, Solemya velum, and many very large Mercenaria mercenaria.  There are a few small rock outcrops with Littorina littorea and a few Urosalpinx cinerea.  Right at the shoreline, small hermit crabs carry Littorina littorea shells, and on many of these shells you will find the small, dark colored Crepidula convexa.  You never find them on live Littorina, only on those carried by hermit crabs.  Large Crepidula plana are often found in dead Busycon or Polinices shells. The town has cleared the area of most small rocks, but before that Anomia simplex was common.
* Fort Phoenix in Fairhaven - this is a state park, but no-one objects to collecting.  The bathing beach area is not productive, but the rocky area below the fort (an old Revolutionary War structure with huge iron cannon still in place) is good.  Beach collecting here is very good.  Crepidula fornicata are amazingly abundant.  There are Littorina littorea on the rocks, and Mytilus edulis and Geukensia demissa in rock crevices.  The water is not as clean as on the cape, since this is just outside New Bedford harbor.  Turning rocks, and sifting sand under rocks is the most productive method here.  There is Bittium alternatum, Urosalpinx cinerea, Eupleura caudata, Anachis avara, Mitrella lunata, Nassarius trivittatus, and sometimes Seila adamsi.  Common bivalves include Anadara transversa, Nucula proxima, Petricola pholadiformis, Anomia simplex, Mya arenaria, and sometimes Yoldia limatula.
This getting a bit long, so I'll write a separate posting concerning a couple of north shore sites.
Paul M.