If you're on the bay side in Brewster, you're already on the famous Brewster flats, and the bay side is still fishing well this time of year. The water is cool and sand eel numbers are way up
The Paines Creek area in the western part of town is probably the most popular with fly fishermen, but but anywhere on the flats where there's bottom structure such as gullies and channels is a good spot to target cruising stripers.

The tides on Brewster flats are dangerous and there can be fog at any time
The drop off at low tide is as much 3/4 of a mile out, so carry a compass and take readings as you work your way out. Those same gullies that you fished on the dropping tide will have water that might be too deep to wade on the way back.
Once you notice that the tide has turned, start to work your way back, don't wait
Sight fishing is best done with a high sun and calm conditions. The temptation is to wade deep . Try to wade no deeper than knee level, and only cast to sighted fish.
If conditions are not ideal, blind casting to deeper areas, like those channels or the drop off will work

Like I said, there's a good population of sand eels this year and that's what the stripers are looking for.
Flies that are long and skinny and stay near the bottom will work best.
A Clouser minnow style fly of 4 to 6" with smallish eyes would work great, but anything skinny that sinks quick will do.
A crab fly may be important too.
EDIT>>>> I just read a report on an other board. It seems that are lots of small sand eels about now too
1 1/2 to 2"<<<
There's a lot of stuff written on the web about northeast flats fishing and some of the best are in the FAOL archives
here's one to get you started
http://www.flyanglersonline.com/feature ... art59.html

Also, don't hesitate fishing a high tide at night right off the beach. If there's a creek with eel grass, all the better
Silversides spawn in the grasses on full moon nights.
Right place, right time....you'll never forget it

Here's another shop that might be closer to where you are staying
http://www.goose.com/
These people have been very helpful to me in the past.
They rent kayaks too, and can direct you to some good spots in the Nauset Marsh area (hint, hint )