Hi Max,
Dustypuss is no doubt wise to warn you about taxi fares but they're not always expensive. I did a day trip to Rome, a couple of months ago, and missed the bus back to Ciampino airport. I shared a taxi with someone else in the same position and the fare was only �30 (for a journey taking nearly 40 minutes). That's a great deal cheaper than London cabs, where the short (2 mile hop) from Liverpool Street to Waterloo costs around �15.
However, by far the cheapest way to get around is with a BTI or using BIGs:
A BIG (biglietto integrato gionaliero) is a day ticket for unlimited travel on the buses and metro. It costs just �4
A BTI (biglietto turistico integrato) is a 3-day ticket for the buses and metro. It costs �11.
The weather forecast is quite good
http://uk.weather.com/weather/10day-Rome-ITXX0 067
so you might want to spend some time on the beach. If so, take a southbound Line B Metro train, for 4 stops, to Pyramide. As you come up the steps out of Pyramide station, you come into a surface rail station. This appears on most transport maps as 'Ostiense' but, confusingly, it's got a different name at the station (and some maps inside the carriages have yet another name for it!). Anyway, that's where you catch the train for the 30 minute journey to Ostia Lido. (The BIG & BTI tickets are still valid on this train). Ostia Lido isn't the greatest seaside resort in the world but the beach is pleasant and there are some really good (inexpensive) restaurants.
For other ideas, try the 'Rome in 48/96 Hours' link, here:
http://www.romaturismo.it/v2/en/main.asp
Anyway, you won't see me on CB tonight, 'cos I'm off to bed early. I've got the alarm set for 0340