The port of Piraeus is the main port of the country, feeding Athens with ships filled with traders, goods and tourists. If you travel to Greece, you've probably arranged a trip to the islands, so you'll definitely pass from the harbour, since this is the start point for all ferries itineraries.
The port of Piraeus is the largest passenger port in Europe and the third largest in the world, servicing about 20 million passengers annually. As it is expected from a busy, working port, Piraeus is always crowded, not only with tourists but also with working people. Almost all shipping companies have their central offices at the port, along with hundreds of other businesses, banks, shops etc. Keep in mind that Greece has a long naval history and attracts maritime industry professionals from all over the world.
Piraeus is also a significant shopping center for those who live at the southern part of Attica and the home to innumerable brands and stores. Moreover, the whole district is full of nice cafeterias, taverns, restaurants, bars and clubs, many of which offer great sea view. Especially in Mikrolimano area, you'll find a series of traditional restaurants with fresh fish and typical Greek delicacies which are served with "ouzo" and wine.
The terminal station of the electric railway (Line 1, "green" line) is situated opposite the port's gate. There's a pedestrian bridge connection indeed, taking you from the train platforms exit to the the sea front. If you have time, pay a visit to the ISAP Museum, which is lodged inside the terminal and is housing objects, books, photos and documents from the beggining of the Electric Railways Company till today.