We spent three weeks on the Cape this summer. My family especially likes the Chart Room, in Cataumet (near Falmouth, Centerville, etc.) Very cool environment and great food (seafood and otherwise). But...I'm not sure if they're open this time of the year. You should check first. I don't know where Brewster is, so I have no idea whether or not you'll be close or far.
If you make it to that part of the cape, you should also go for dessert to Four Seas Ice Cream in Osterville. It's been discussed elsewhere on the site. Again, make sure they're open.