We LOVE Hershey Park! It was my family's very first vacation back in the early 90's when my brother and I were little. We've been back five times since and the whole area is one of my favorite places in the world. We always stay in Lancaster, about 45-minutes away from Hershey. It's a GORGEOUS area with lots of stuff to do. I can't recommend the Willow Valley Resort enough. It's huge, clean, beautiful and has a whole water park for the kids.
http://www.willowvalley.com/
In Lancaster itself is Dutch Wonderland, a cute amusement park for kids. There's also Kitchen Kettle Village, a free little amusement area with lots of shops and restaurants selling Amish goods. You can take buggy rides through the countryside, visit the farms and Amish homes, and time scenic drives through the fields. It's really gorgeous. I know none of that sounds too action-packed, but my brother and I LOVED it when we were younger, and still do. We haven't been back in 4 years since we started going to Disney yearly, but I really do miss it. My friend and I are driving out for the day this summer just to do some of the Lancaster attractions.
If you're coming from the east, Crystal Caves is in Kutztown, PA, about an hour before Lancaster. You go down into the caves and get a tour of all the formations. It's so cool! The Strasburg Railroad is also nearby for old-fashioned train rides through the countryside. In Hershey itself, there's Hershey Park and Hershey Chocolate World. Chocolate World is free and includes a fun little ride through a recreation of the Hershey factory ("Hershey chocolate, Hershey chocolate, it's a Hershey chocolate world..." That song brings back so many memories!), stores and a cafe. The park itself isn't that big and can be done in a day.
As you can see, I'm a big fan of the area

It's a great vacation!