The old train shed is beautiful, and the headhouse is well maintained (or at least it was at my last visit). A lot of the old stuff from the station has been saved/reproduced, including train signs by the tracks. And the trainshed itself (the largest of its type?) is really cool to look at, however, one can not go inside. The park itself is also great, and when you're done, and if you're feeling up to it, you can watch trains on the National Docks branch, which runs just behind the TPK extension.
Also, the Liberty Science Center is nearby if you need somewhere to stash less-than-interested companions