If you plan to go to the Vineyard, go early. The parking lots for the ferries in Falmouth fill up fast on nice days and you don't want to wait for the final ferries of the day to leave the Vineyard because it will be a shitshow. The island is pretty small. If you're game, you can rent some bikes and literally ride around the island. Otherwise, most of the businesses are primarily centered in the area where the ferry dumps you off.
Dennis beach is nice (faces the inner part vs. the open Atlantic, so when the tide goes out, there is like half a mile of sandbars you can walk out on), but there is some endangered bird that made a nest and they recently shutdown most of the parking recently, although that should be cleared up by the time you get to the Cape. The National Seashore beach in Barnstable is nice as well, although avoid the hiking trails because they are full of ticks. Speaking of which, the Cape is full of ticks. Check yourself frequently if you spent any time near lawns or woods. Beach should be fine, the asshole green flies that bite are the only concern there usually lol.
The Whydah pirate museum in West Yarmouth is pretty cool. It is pretty new museum /w National Geographic support and has a ton of awesome artifacts and treasure from the Whydah, which is the only pirate ship wreck ever found. Not to be confused with the Whydah museum in P-Town...that was the first museum they made back in the 80s, but it doesn't have much besides some coins and a cannon and all the good stuff is at the new one. The new one has everything from the anchor, to cannons, to a fuckload of gold and silver, to weapons. They are still doing active excavation and have an entire room with artifacts from the wreck still encased in the concretions from the seafloor.
P-Town I've never been a huge fan of, but lots of people love it. It is waaaaaaaaaaayyyy at the tip of the Cape and the road eventually narrows to a single lane deal that gets really congested at peak times. Another "go early/leave early" type of place...lots of restaurants and bars and shops. At night it can get pretty rowdy, which can be fun if you're into that, but it is less fun when all the drunks try to drive home on that narrow ass road at night lol.
Hyannis has a bunch of stores and restaurants. Oddly enough, the best Mexican food I ever had is at a little hole in the wall place here called Mi Pueblo. It is kind of hard to find because it is down this alleyway near the former Hojos (i dunno what the fuck it is now, I just remember when it used to be a Hojos my grandmother used to be the manager for back in the 60s. Fun fact: When JFK won the election, Jackie Kennedy came in and tried to get away with not paying for the food they ordered for the party. My grandmother wasn't going to take that shit and told her she didn't care if he was the president, they needed to pay lol). There isn't any big signage, but the food is damned good.
IDK where the best lobster roll is. We used to go to the Rawbar in Mashpee because they gave you a shitload of lobster, but the roll is like $28 (used to be $14 when no one knew about the place back in the 90s) and pretty sure they just use frozen vs fresh lobster meat now. I'll eat a lobster roll, but I'm not one of those devotees that go crazy for them. My aunt in Maine lives next door to a lobsterman that would sell them a few cheap when he got back from his traps, so I ate it a lot as a kid and I got kinda sick of it lol.
Oh ya, and do mankind a favor and read up on how to drive in a rotary if you are unfamiliar with them. Every year the Bourne Bridge rotary is a shitshow because all the tourists don't know how to use one and it backs up traffic for miles during peak times lol.