Bonn to Venice is 12 hours by train, and wastes a day. There is a 15 hour night train but it's not a non-stop. You have to change trains in Cologne and in Munich. Waiting on a train platform at 3AM is not a great experience. Flying from Bonn to Venice you can then take a train to Florence, make it your base and do a day trip to Sienna, another to San Gemignano. Then continue by train to Rome and leave from there. If you want to see the Tuscan country side then as JMN57 said, you will need a car. Your itinerary will start getting packed and complicated, because anywhere but in the countryside a car in Italy is a liability. You said you've never been to Italy before, so I'd keep it simple. Three days in Venice, two in Florence, with one used for a side trip to Siena. The remaining five days in Rome, with perhaps a side trip to Orvieto or Ostia Antica. That's plenty busy for a first timer. No cars, nor rush, no real hassle, as long as you can get a decent hotel on this very short notice. If you want to stay in small towns you'll need a car.