We took the Proastiakos to Corinth - the new town is nice enough if nothing spectacular, but you can get to the ruins of old Corinth (we ended up getting a taxi from the station) and that is well worth the visit if you haven't been there already - and also the Corinth canal (and the Diolkos which was there before it) is impressive. Alternatively we looked at going to Chalkis/Chalkida which seemed worth the trip but we ended up not being able to go.
If you aren't set on going by train and the timing works, a quick trip on the ferry to either Aigina or Poros is also worthwhile, both are around 1h from Piraeus.