Originally Posted by
KLouis
Ostia Antica would be really fine (half a day, though, a whole day might turn the kids to terrorists), but there is no metro to there!
Take the Metro to Magliana and change onto the Ostia (C. Colombo) line to 'Ostia Antica'.
On the way back why not stop off at EUR Fermi and take in the rather quirky Mussolini grand vision architecture. Few tourists visit EUR, but in my opinion it's worth stopping off.