The best way to see the Amish in Lancaster is to get off the main highway onto the side roads (but you don't have to go by bike! ). For example, route 30 is a major road through Lancaster, but if you take route 340, which runs almost parallel, then you have a better chance of "Amish sightings." It's fun to pick a side road and just wander. You can't get seriously lost, because eventually you will come out to a main road.
Also, the farm stands will still be open in early September, so even if you aren't in the market for a bushel of fruit, you can stop and shop and see Amish farm life up close.