Fun places to take kids on a day trip?

Every Summer my grandparents take my two cousins on day trips and usually ask me to tag along to help. I love my little cousins so I really don't mind going. I am 14, my boy cousin is 8 and the girl is 5. My age doest matter because of course we always go somewhere age appropriate for them. The last two years we've gone to the Camden Aquarium because they both reaaaaallly wanted to and love sea animals. We live in N.J. So anywhere in NJ, NY, or PA would be fine...just as long as it's not in NYC. Thanks!