There's no better way to really see and enjoy the Cape and Islands than by bike. The Cape's roads are emptier in the fall, and the weather's a bit cooler, too. Pedaling is simply more pleasant. We do our inn-to-inn bike tours on the Cape and Islands either before July 4th or after Labor Day. If you want to set up your own bike tour, drop me an email. I have several great itineraries from past trips.
Cape Cod is blessed with a number of wonderful multi-use recreation trails that make for great biking, especially after the crowds have thinned. The two service roads along the Cape Cod Canal (nae.usace.army.mil/Missions/ Recreation/CapeCodCanal.aspx) are about 7 miles each. The Shining Sea Bikeway in Falmouth (woodshole.com/documents/bikewaymap.pdf) is 10.7 miles. The Cape Cod National Seashore has several trails (nps.gov/ caco/planyourvisit/upload/2012biketrail web.pdf), including a wonderful 5.5-mile loop on the Province Lands at the very tip of the Cape. But the best of them all is the Cape Cod Rail Trail (mass.gov/eea/agencies/dcr/ massparks/region-south/cape-cod-rail-trail.html) and its side spur from Harwich into Chatham, the Old Colony Rail Trail (town. chatham.ma.us/Public_Documents/ChathamMA_Bikeways/OCRT_Poster09_11x17.pdf).
I can think of a whole lot worse ways to spend a few days on the Cape than biking all these trails. If you want more riding, it's easy enough to bike to the end of the Shining Sea Bikeway in Woods Hole, catch a ferry out to Martha's Vineyard (mvy.com/Visitor_Information/ Biking_ on_the_Vineyard.aspx) and ride the miles of flat, dedicated bike paths on the eastern end and around the center of the island. The western end of the island is much hillier and the roads are narrow and curvy. With summer traffic, they aren't much fun to ride, but in the fall (or spring) they are pure joy.
Finally, if you are a mountain-biker, explore the maze of singletracks and fire roads around the kettle ponds in Wellfleet and Truro. This is strictly a do-it-yourself exploration, but you'll see a side of Cape Cod most others never even know exists.
Kayak and SUP
If you bring your own paddlecraft (many rental shops close early), Cape Cod is heaven in the fall. There's a list of "official" boat launches (cc-waterweb.com/ramps.htm), but with kayaks and SUPs any beach is your launch pad to fun.
Be sure to try paddling the wonderful freshwater ponds on the Cape. In the fall you often can have the whole pond to yourself.
If you own a seaworthy kayak, every harbor, marsh and tidal pond along the coast is fair game and many days you can safely take to the open ocean in calm seas. Just be aware of chang ing conditions and stay within the safety margins of your boat and your own ability levels.
If you need to rent kayaks or SUPS, or want to do a guided trip, be sure you call or email ahead. Many places go to weekends -- only right after Labor Day, but I've often found the proprietors more than willing to go out weekdays with advanced notice.
From personal experience, I can recommend kayak or SUP adventures with these companies:
Cape Kayaking (capekayaking.com; 508-247-7402)
Cape Cod Paddle Surf (capecodpaddlesurf. com; 508-737-8572).
Goose Hummock Outdoor Center (goose.com 508-255-2620).
Island Spirit Kayak (islandspiritkayak.com).
If the proverbial "walk on the beach" sounds lame, how about a real hike instead? The Cape offers a number of genuine hikes:
The Great Island Trail (cctrails.org/lng%20 greatisle.htm) in Wellfleet is the longest and most remote "official" hiking trail on the Cape. The views of Cape Cod Bay are expansive and breathtaking (especially at sunset). The distance and soft sand make this a challenge -- you'll feel your glutes and hamstrings.
The basic dry-feet hike is about 7 miles, round trip. At low tide, you can reach Jeremy Point, which is underwater at high tide (boatma.com/tides/Cape-Cod.html), and loop around the perimeter of Great Island for an honest 10 miles. Go on a rainy day or mid-week in the off season, and it might seem like you are the last person standing at the very end of the earth. Nice getaway.
Other Cape/Islands long hikes
Provincetown: Cape Cod National Seashore Pilgrim Heights to Race Point Light, cctrails.org/lng%20racept.htm
Wellfleet: Wellfeet Bay Wildlife Sanctuary, massaudubon.org/Nature_Connection/Sanctuaries/Wellfleet/index.php
Martha's Vineyard: Chappaquiddick Island Cape Poge Wildlife Refuge. Dike Road to Cape Poge thetrustees.org/pages/ 286_cape_poge_wildlife_refuge.cfm
Nantucket: Coskata-Coatue Wildlife Refuge in Wauwinet, to Great Point Light and Coatue Point: thetrustees.org/places-to-visit/cape-cod-islands/coskata-coatue.html