I don't see historic Philly as a good option with a little one unless she sleeps in the stroller while you walk around - not much to interest her unless you are going to museums with children's centers, and you can do that in Boston/Providence. It's also very hot in Philly in the summer. I have friends in Center City and LOVE the area, but your little one is not old enough to appreciate things. I think it's great during April vacation, which is a Mass/RI thing and not universal, so you don't always have the crowds.
There are many towns in Lancaster County - we did it some years ago in a rented RV and had a fabulous time. My family comes from central PA so it's definitely near and dear to my heart. Our son loved it, and the campgrounds offered small ponds/lakes, covered bridges, and so on, plus cheap campsites. You can do small cabins too, and there's a lot to be said for the quiet. Many of the local people (Amish & Mennonite) came through selling home made pies and breads and it was wonderful. We got lots of great info from AAA. Your daughter is a little young for the Strasburg railroad but it's fun. There are some great shops and museums if you and your husband are into that, but it depends on your child's nature, sleep schedule and so on. Try not to rush the theme parks - your child is only 2 and won't remember much, may be overstimulated, and so on - takes the pleasure out of paying that kind of money. So choose wisely. You might be better off postponing it for a couple of years and trying something else closer to home while she's so little. When our son was little, we just went to Cape Cod (no more than a few hours) or day trips to the Mystic Aquarium and the Boston museums. That was more than enough car travel for him.
I'm not trying to discourage you, just help you choose a realistic vacation for this year.