Where's a good place to take teenagers on vaca in August, within 6-7 hours of Western NY?

I'm a single mom of 2 kids. My son will be 20 and is in college; I see him for what I have realized is something like hours per year, not even days. My daughter will be 16; she lives with me and we get along well.

I'd like to rent a cottage w/ 3 bedrooms somewhere they will both find entertaining for a week at the nd of August so that I can spend some time with them. They are both gregarious and outgoing, and I will invite each to bring a friend.

I'm just not sure of where would be fun for them. It has to allow them some independence, even the 16 year old. Not too expensive. Maybe a little night life but fairly safe of course. A beach area might be nice too. Any suggestions, anyone have any successful vacations of this sort?