Siena, as much as I love it, is some distance from Rome. I'm not sure I'd recommend riding around on scooters in Italy, it looks cool when Audrey Hepburn does it in the movies, but I think it might perhaps be an unnecessary risk.
Evenings in Italy are all about the
passeggiata, the evening stroll where everyone hangs out on the piazza. (Actually, I remember the Siena passeggiata being a really
good one, but I had a great time in Rome as a twenty-something, too.)
As I recall, the main areas of Rome tend to be Via Veneto, the piazza near the Pantheon (The Pantheon is a
very special place) or the Trastevere.
If you plant yourself in one of those spots, the nightlife will find you.
Just be a bit careful on the #64 bus.