travel to Siena and Assisi

ralph,

The easiest travel method from Florence to Siena is the SITA Bus, and the station is very close to Firenze SMN rail station, so very easy to reach. As I recall, the Corse Rapide (express) Bus takes about an hour each way, and you'll be dropped in Piazza Gramsci in the centre of town, rather than the rail station at the bottom of the hill. Don't forget to validate your ticket before boarding the Bus!

The trip from Florence to Assisi will be as short as 2.5 hours each way by train, and from Orvieto as short at about 2-3 hours (depending on which train you use). Of the two, I'd probably make the trip from Florence as there are a few direct trains available (no changes). There's a direct departure at 08:02, arriving 10:40 and that's probably the one I'd use. On the return, there's a train departing Assisi at 19:17, arriving 21:48. The trains on the Florence - Assisi route are mostly Regionale, so just buy your tickets locally when you arrive in Italy, but DON'T forget to validate them prior to boarding the train on the day of travel.

When you arrive in Assisi, you'll still have to take the "C" line Bus up the hill to Piazza Garibaldi. You should be able to buy tickets for the Bus in the Tabacchi shop in the station. I'd suggest buying one for the return trip as well. From the Bus stop it's a fairly easy walk to the sights, although the Basilica of St. Francis is at the far end of town.

It may be more convenient to take the bus from the Assisi station to Piazza Matteotti, from there all the sights (except the Arena and Rocca Maggiore) will be a downhill walk.

Florence is the better base.