Depending on where you start adn how long you have, there are a few possibilities.
Strasburg RR and the Pensylvania RR museum are located in Strasburg PA, not that far east of Philly. Strasburg runs steam, the museum has a large collection of stteamers on display.
Steamtown is in Scranton, a ways off the turnpike. They have a large collection on display and I think run some. I have not been there in some years.
The East Broadtop is in Orbisonia PA. They are currently not running I think. they have 6 narrow gauge steamers. It is a tourist line but all the origianl buildings are there along iwth quite a few original freight and passenger cars. Not too far off the Pike.
there is always the Horseshoe Curve. No steam, but should be some nice action. Altoona itself might be good too and there is a museum there also.
If you are going to get off the turnpike and continue west on I70 toward columbus, there is a very nice streetcar museum very near Washington PA. they run some cars at least every weekend. A couple of mile ride I think.
If you really want to get off the path and chase some modern stuff, go down to Sandpatch on the old B&O or Mann's Curve in Maryland. All CSX now, but neat spots. Probably not easy to get to as a side trip across the state. Just a little too far.
If you are going out the turnpike to Ohio, you can get off at exit 18 (I think) and right next to the turnpike there is a little town called Homewood. the old PRR comes up out of the Beaver River valley right there and makes a sweeping curve as it turns west to cross Ohio. that is now the NS main.