When my kids were very young, we spent the week+ of our family vacation at the Jersey shore and/or Lancaster/Hershey area. Dutch Wonderland in Lancaster is an amazing amusement park for the little ones. FOr a few years we split the time between the Lancaster area and the shore. We would stay in Lancaster and/or Hershey for about two nights and, as far as amusement parks go, just do Dutch Wonderland back then. Then we would spend a day at the Hershey Zoo and Botanical Gardens, which has a butterfly house, something my kids always enjoy. We also have done the caves that are right near Hershey...although it was a lot of steps when I was tired and pregnant with #2! :) We went to the Hershey "Factory" as well. Then, at the shore, we relaxed at the beach and went to Ocean City, a great town for kids, and to Lucy the Elephant...just outside when they were that little, inside when they were a little older and could appreciate it. We always stay right by the beach, in a less touristy town, but we tend to go to Ocean City at least once a summer.
We didn't start taking "bigger" summer trips with the kids until my youngest was 3, and we went to the Mystic, CT, area for 7 days that year, and it was absolutely fantastic. We stayed in Mystic but also visited some of the towns nearby, including Groton, where there is a submarine museum. There's a lot to do up there.
Since that vacation went so amazingly well, we have taken a "driving vacation" every summer since. All have been fantastic; our family's summer vacation is my favorite week of the year!
(I don't know if you're thinking summer or another season. We do fly to Florida every spring, to visit one set of grandparents, and my kids always have enjoyed that, too...about an hour north of Miami...with a lot for the kids to do.)