One of my favorite parts of living in Lancaster how easily accessible hiking on the Susquehanna river is and one of my favorite hikes originates from the Pinnacle Overlook in Holtwood.
Holtwood is about 20 minutes south of the city and easy to get to driving south on Route 272 until you reach Holtwood Road. You take a right and signs will take you to Pinnacle Overlook.
Pinnacle Overlook is located on the Holtwood Dam preserve and is essentially a scenic overlook on the river. There's a variety of trails to chose from but I recommend "Kelly's Run" which is marked in orange. It takes you down the side of the valley and up a stream bed (which isn't empty, so wear shoes you don't mind getting wet) and then back again.
I like this one because you actually get a workout: crossing the stream requires some acrobatics if you want to stay dry and to get back up the valley you need to do some climbing and scrambling but are rewarded with some great views and several swimmable pools.