This is a lovely area & we have stayed here for many years. The private bay beaches are great; at low tide you can walk out to several sand bars(depending on the year!)
You can walk to Chapin's up on Taunton Avenue; and there are several restaurants in Dennis along 6A-- Sargo's Cafe, Red Pheasant and Summer Stock (off 6A)
Mercantile's on route 6a is great for breakfast and you can pick up sandwiches for the beach there as well-
Dennis public market is great for groceries, meat & deli too
One caveat however, we found last July that unless we got up early ( 6:30 - 7:30am) and were back by 8 or so, we could run into a 20 minute wait to get back to our cottage (and this was after taking side streets so that we could access Beach Street closer to where we were staying); cars were lined up down Beach Road to get into the Mayflower Beach parking lot. Beach Street is the only way to access where we were staying (and Dr. Bottero drive) from Route 6a. Traffic near Mayflower Beach in the morning (even if not an optimally sunny day) was terrible & prevented us from going out after 7 or so. We did not leave the beach at noon time, but I have read that the traffic last summer approaching Mayflower was difficult at times even during the day. We can certainly speak to the fact that you may well run into a long wait to get down Beach street in the morning.