Realistically, Harrisburg to Lancaster is 40 minutes to an hour on Route 283 slab, which is not a particularly exciting ride, just average four lane stuff.
A nice alternative to come from Harrisburg is to take route 441 from Middletown, which runs down the east side of the Susquehanna River, past the 3-Mile Island nuclear plant, and down through Marietta and Columbia. It will take longer, but it's a nice ride. It eventually becomes River Road in Washington Boro. Route 23 is a nice 2 lane running from Marietta to Lancaster City.
The only state park in Lancaster County is Susquehannock, which I've never visited. The Strasburg Railroad and the Train Museum in Strasburg is pretty cool if you are into trains, and the Farm Museum in Landis Valley is interesting.
Strasburg Rail Road - Lancaster County, Pennsylvania
Landis Valley Museum - Pennsylvania German Heritage - Lancaster County Tourism, PA
Officer Friendly gave you some great stuff, and he is very right about the no photos stuff. The Amish are great, but they are definitely a group born of a religious conviction first and foremost. The surprising thing for many people is to find out that they have a great (if not different) sense of humor and they value family, hard work, and privacy. They don't shun modern technology, but they accept or reject new things based primarily on how it will positively or negatively effect their community and culture. Amish buggies are gray. The black buggies are plain sect Mennonites and (I say this as a Mennonite myself though not plain at all) they are not as nice as the Amish generally speaking. An interesting note on the Amish is that while the success rate of all new small business is about 50% at best, the success rate for new Amish small businesses is over 90%. Not bad for a group who rarely goes past the 8th grade.
I apologize if my post is rambling aimlessly. I'm a recovering from several hours of violent sickness last night. I'm thinking food poisoning. My head is a bit foggy today. I hope you have a great time here. It sounds like you will have fantastic weather.