Ostia is spectacular, the picturesque remains of a working port town cover an enormous area of red-brick and marble ruins on the banks of the (now scarcely visible) river Tiber. It gives much more of a feel for life in the Roman empire than yet another nutty emperor’s palace. (Not that some of those aren’t pretty spectacular too.)
Wandering along the deserted streets with weeds growing through the flagstones, sand blowing over the mosaics, and long grasses and wild flowers in the ruins, often shaded by umbrella pines, one also gets a sense of the “Romantic” that drew the 18th and 19th century travellers to Italy to write bad poetry about the fall of empires etc etc.
To get there from Rome you take the train (or is it a metro – it doesn’t spend much time underground?) from the railway station variously described as Pyramide (on the outside of the station building), Porta Ostiense (on the map), or Porta San Paolo (within the trains themselves). We found it easy and clean, takes about 20 minutes and the regular €1 metro (ATAC) ticket or Rome pass seems to work. (And see recommendation 10 on the Blue Guides’ list of things to do in Rome.)