We have gone to Cape May Point for the last 3 summers. It is such a quiet, beautiful spot. There is a cute little state park with a lighthouse and a great nature program for kids.
We took the kids to Beaches in Negril, Jamaica 2 years ago and we all loved it. Very expensive, but totally family oriented, great food, excellent childcare available during most of the day and evening included in the price, and absolutely stunning beach. We'd like to try the one in Turks and Caicos sometime.
Hawaii was an excellent place to take kids. There were beaches that had large sand bars protecting them so kids could swim like in a pool. So much to do there and so beautiful.
We took the kids to Jazzfest in New Orleans twice. There is a great kids area with a multitude of activities and entertainment. The music and the food are outstanding. The crowd is a very family oriented bunch. It was not rowdy in the least. There is also alot to do in New Orleans with kids. They have a great zoo and aquarium with a ferry between them. They have many small historical museums, a childrens museum, and interesting touring. The French Quarter is fine if you skip Bourbon Street.
San Francisco is another fun city for kids. Riding the street cars, walking the variety of neighborhoods. We rode bikes across the Golden Gate bridge with a kid in a Burley trailer all the way to Tibouron and took the ferry back to Fisherman's wharf. There are great restaurants including an excellent Chinatown. Just outside of SanFrancisco there are many other wonderful places to go. We loved Point Reyes National Seashore.
Before the kids we went on a trip to the four corners in the southwest and saw Taos, NM, Moab Utah with its amazing mountain biking and beautiful national parks, Mesa Verde, Bryce Canyon, Zion national park, the north rim of the grand canyon, monument valley, etc... We are waiting for our kids to get a little older, because this trip requires a lot of driving to get from one place to the other. I think they would enjoy the sites now, but their long drive tolerance is low.
We love Mount Desert Island in Maine. We just never have two weeks in the summer anymore to make it work with the long drive from Philly.