We were coming on Sunday. Now I see everything is closed. Is Sunday a bad idea?
All Amish and Mennonite businesses will be closed on Sunday so if those were the reason for your visit; unfortunately, Sunday is not the day to visit.
The outlet centers are open on Sunday as well as Landis Valley Village and Farm Museum. The Amish Village and The Amish Farm and House are also open on Sunday but note these are tourist "recreation" attractions, not actual Amish residences. The Strasburg Railroad and Railroad Museum of Pennsylvania are open on Sunday.
As fly eagle notes there will be plenty of tourist attractions open, but you will not be able to interact with Amish people or shop at Amish or most Mennonite businesses on Sundays (Although you will see plenty of Buggys on the backroads going to or from church)
All farmers markets and almost all farmstands will be closed as well
To see the many things you can do go o the very good tourist website www.discoverlancaster.com