in Rome the are a lot of places...need to have time ..Also people of Rome have difficulties to see all...Because many building are private..some time also foreign states have nice palace with frescos etc.
To know some
http://www.italian-art-notes.com/2014/07/in-rome-inhabitants-are-also-tourists.html almost 5 years ago
in Tivoli are nice the villas, any people visit the castle...Villa Adriana, Villa d'Este, Villa Gregoriana ...To go in the center of Tivoli it is also the train by Termini station ..need 35-40 minutes and you may walk for Villa d'Este and Villa Gregoriana...Villa Adriana is a bit hard to reach...
To go to Ostia Antica is sufficient to take the metro to the station closest to the Pyramid.
But these places you can find some tourists. Places are sufficiently known.
You find in wikipedia: Villa d'Este, Villa Gregoriana, Villa Adriana and Ostia Antica. almost 5 years ago
For Tivoli better you see this site: http://www.tivoli-iloveyou.com/en/
It is the site of local organization..
For Ostia antica just take the subway. On the subway you can find many inhabitants of Rome who go to sea. After Ostia Antica, there are the stations of the beach of Ostia. almost 5 years ago
in Rome you may see some nice place unknown to many tourist
http://www.villamedici.it/en/ near Piazza di Spagna
http://en.wikipedia.org/wiki/Basilica_of_San_Clemente no far Colosseum
http://www.lazio-directory.org/mausoleocostanza.html via Nomentana, need take new metro
http://www.lazio-directory.org/mosaiciprassede.htm near Santa Maria Maggiore almost 5 years ago