i lived in boston for about 4 years and from early May to October there is excellent fishing here. mainly striped bass (stripers) are the targeted fish and it is idiocy to target any other species during this time as when the bluefish come in they will be in the places the stripers will be. the only problem witht htis is you will need a metal leader for bluefish as they have very sharp teeth, stripers can be thumbed when you catch them.
they can be caught all over the capeduring the early and late months of summer although when the really hot weather comes in the fishing falls off slightly, what the locals call the summer doldrums. during this time night fishing with bait can still be productive on the outer cape beaches from nauset to provincetown. let me know where you'll be staying. One word of warning though the locals are not your typical Americans and are big into their bass fishery and its conservation (stipers were almost overfished to the point of extinction in the 80's and very early nineties, leading to a complete ban on fishing for stripers for a few years). you can only keep one fish per day and it must be over a certain lenght. last year the keeper limit was 30 inches and there are heavy fines if your caught with undersized fish by the authorities or a beating if locals catch you. i envy you going there for the summer, as far as fishing goes it 4/5 amazing months. i will give you 2 tips
1. save about USD250 for the end of your trip (late august/early september weather permitting as some really great species move into the area such as yellow tail tuna, blue fin tuna & mahi mahi. if you can reserve a spot on a charter its worth the chance to target these species and if you dont get the above they'll take you to spots to target really big stripers
and
2. visit
http://www.reel-time.com - it provides a huge amount of data on the fishing in the entire east USA but the main sections are dedicated to NE & cape cod. its mainly targeted at flyfishermen (which is my thing mostly) but all the info is apt for whatever you're doing, fly, spin, bait. the forum provides up to the minute info from all types of fishermen on where the fish are & what they're taking. you can do searches etc for where your fishing to find out historical info, dont be turned off by the US style of posting 2 paragraphs where 1 sentence will do!
another tip- look for the charter boats links on the website & call them as many of them require deckhands for the summer and if you're not too late you could get a plum job - cash in the hand + tips + decent hours, not waiting tables all night.