We LOVE Goose Rocks Beach in Kennebunkport, ME. Gorgeous beach. You can walk out to the rocks at low tide which is fun. A short drive into Kennebunkport for shopping. The nearby village of Cape Porpoise has some great restaurants.
http://www.visitmaine.com/attractions/outdoor_recreation_sports_adventure/beaches/beaches_by_region/maine_beaches/goose_rocks/

A couple of things to consider, especially if you are from the South.

1. It is good to recognize that the areas you are talking about from mid-coast Maine (not as far north as Portland) to the Inner Cape often have beautiful wide sandy beaches and water that is way too cold to swim in, especially before mid-August.

2. Some areas that are otherwise desirable are honky-tonky, e.g., Hampton Beach. This is fine if it is what you want, but you need to know it. You are unlikely to find nature and boutiques in the same immediate area, though the Outer Cape may offer the best bets.

3. Weather in New England before Fourth of July can be chilly and/or foggy. This is nice if you want to do non-water tourist things (go to Portsmouth or Portland or Marblehead), but not great if you want to spend your entire time in the water or getting a tan. But you seem to know about that.

4. The closer you are to July, the more likely you are to encounter biting greenhead flies, particularly on some of the most beautiful beaches north of Cape Ann (e.g., Cape Ann).

5. All that said, the best places to rent for families would be the Outer Cape from Truro or Wellfleet down to Chatham or along Nantucket Sound. The South Coast beaches of Rhode Island can also be beautiful. All these areas have lots to do.

Consider the beaches in R I. There are some lovely ones in places like Misquamicut, Charlestown, Metunic, Moonstone, etc. and the water isn't quite as cold as in Maine.

The damage suffered from Sandy should be cleaned up by summer.

Consider Orleans on Cape Cod..both Atlantic Ocean and the calmer, warmer Bay. Or a town on Nantucket Sound like Dennis, Yarmouth warmer, calmer waters..

If you want to consider actully going in the water, the water temps are considerably warmer from the south shore of the Cape down to Rhode Island. The east side of the Cape is the most scenic, being mostly National Seashore beaches, but the water will be cold and possibly rough. The bay beaches (inside of the arm) on the Cape are quiet, and family oriented, but again can be chilly. The south shore of the Cape is the most commercial, with lots of motels, malls, mini golf, fast food, etc. But the beaches are quite bearable and calm. Falmouth is a nice town to be based in for this part of the Cape.
If you are not too concerned about cold water, York Beach Maine is a great old timey beach town with a zoo, nice beach and lots of kid friendly places to eat.

I'm a big fan of Wellfleet or Eastham, Cape Cod, for families -- you rent on the bayside, where the gentle beaches are, but its a quick trip to the kettle ponds (freshwater) or to the national seashore (oceanside) beaches. These towns have minigolf, bike trails, restaurants, fishing boats (Wellfleet) a drive-in movie theater, and ice cream parlors. They are centrally located between the whale watching boats out of Provincetown and the commercial offerings of the south part of the Cape.

I haven't used them for years, but for the cape and islands weneedavacation.com may have some listings you won't find thru cybor or vrbo or homeaway or flipkey

Thanks everyone! Keep the suggestions coming. Kennebunkport sounds great as well as several towns in Cape Cod. We'll start with Wellfleet and Chatham. I hadn't considered R.I. but as this is my first time to the New England coast, I'd like to aim for the places I've romanticized about in my head.

Incidentally, we live in the Buffalo, NY area. I was thinking that the air temp would be about the same as it is around here - mid to upper 70's for the last week in June. Does that sound about right?

On the ocean, it could be like Buffalo or it could be lots cooler and foggy. Spent many July 4ths in Brewster on the Cape and it ranged from very hot to fine to really chilly, damp and foggy. The ocean weather can be different from inland. You just take your chances with reservations and be prepared for anything.

The beach will almost certainly be cooler than Buffalo.
How important is going in the ocean?
If not the primary thing I vote for Wellfleet. The RI towns are not as special imo.
We need a vacation is a good bet, but I'd do this soon for best choices.

Chatham Mass. Nantucket Sound beaches, Pleasant Bay beaches, Oyster Pond (children's beach. Great shopping, browsing, train museum, shark sightings, seal watches, baseball games, band concerts, art shows, deep sea fishing, fishing off the bridge,lighthouse, history, lobster rolls, ice cream, heaven.

Rockport-Gloucester MA-We rented a very nice house there couple years ago. Cant remember the RE agent but you can google The Golden House in Rockport to look at it.

Apropos of my previous post re Golden House in Rockport, MA Google Golden Cottage by the Sea. I believe the RE agency was Atlantic Realty

The temperatures sound close to Buffalo but it is more likely to be breezy. Water temperatures will be in the 60's on the Cape at that time, so the prevailing SW wind may feel cool. I was mystified when I moved to New England and my mother gave me a cotton sweater. Mom knew best!

We also love Goose Rocks Beach and we own a place in the vicinity. Great wide beach (most of it), nice and shallow for the young kids. Kennebunkport offers lots of shops, restuarants for the adults. The trolley museum is surprisingly fun for everyone. You can head north to Portland (about 30 minutes) and go to a SeaDogs game (AA affiliate of the Red Sox). there's fishing, whale spotting trips, golfing. Just about everything you want from a Maine vacation.