Most town beaches are off-limits to dogs in summer. These are the summer options that appear in the official rules we've verified — each line is condensed from the town's own regulation, so open the linked source for full details, and always follow posted signs and shorebird closures.
Newcomb Hollow, Cahoon Hollow, White Crest and Maguire Landing (LeCount Hollow): the summer ban covers the lifeguarded portions 9 am – 5 pm, 3rd Saturday in June – Labor Day.
Cape Cod National Seashore (Eastham–Provincetown): pets on a 6-ft leash where allowed. Not allowed on lifeguard-protected beaches in summer (may pass through on leash) or in posted shorebird areas (late March – mid-October). Leashed pets allowed on Doane Trail (Eastham), Pilgrim Spring Trail (Truro), all fire roads, and Head of the Meadow bike trail year-round; freshwater ponds Oct 16 – May 14 only.