You can do the parks and the factory in a day. The tour isn't really of the factory where the chocolate is made. It's just a building they made with a 15 minute ride, if that, (in cars with speakers) to show you the process.
I believe they do sell a ticket where you can go the evening of your 1st night there and the entire next day. I'll go look around their site real quick and post back if I see it. I just checked it out and here is what I got:
Preview Plan - Be Our Guest!
Purchase a regular full-day admission ticket for tomorrow, 2-1/2* hours prior to Park closing, and be our guest FREE for the remainder of that evening. (*3-1/2 hours for 11 pm closing)
6 PM Closing - Admission after 3:30 PM
8 PM Closing - Admission after 5:30 PM
10 and 11 PM Closing - Admission after 7:30 PM
You can also buy combo tickets and such, here is the link:
http://www.hersheypa.com/attractions/hersheypark/admission/index.html
Oh, and there are many Giant grocery stores in the general area which sell tickets at a substantial discount. You can only get 1 day admissions at Giant though. I do think you'd be allowed to use that one ticket for the Preview Plan - Be out Guest, plan though.
If you stayed at the Hershey Hotel, they have many beauty treatments, spa, etc... that you can have done. I've never done it (we're local - why stay in a hotel?), but I guess they use chocolate in many of the treatments if not all of them.
As for the Amish, I can't really say I recall too many in Hershey, but DH is saying there are quite a few. IMO, if it's Amish you want to see, you should head east to Lancaster. There are lots of outlets in Lancaster and a short drive north of Lancaster is Reading where there are many outlets.
Dutch Wonderland is also in Lancaster if you're at all interested in that.
BTW: The zoo is included in the price of the Hershey ticket. It's a walk thru zoo. Nothing fantastic (not like the Philly zoo), but it's nice to have that option too. IMO, it's not worth paying a separate admission price for it though, so if you want to see it, do it the day you plan on visiting the park.