This NH four star hotel offers 129 rooms in an enormous building that's well-connected with many buses and right next to the center.
It's very recommendable because of its decor, location and for having a bus stop right below that takes you everywhere. It includes day trips that leave from Sienna from different points in Tuscany (the hotel has information about them), they have the same departure point in the square.
It's not great that one person has to stay for two consecutive days, and that you have to pay for internet in your room (I always say this, it should be a free service), or that they don't speak Spanish, but overall it's a modern, functional hotel and very recommendable.
The Hotel Athena in the beautiful city of Sienna is an excellent option for visiting the city. It has very comfortable, well-equipped rooms. This hotel has spectacular views of the city's mountains. Given the fact that it's in the historical center of the city, it has the proximity to the most important sights in the city, like the Cathedral and the Piazza del Campo.
The Garden Hotel (Siena) has a large open-air pool. It´s located outside the walls, north of Siena. It has a Restaurant, Bar and offers 24 hour reception, daily Newspapers, Garden, Terrace, elevator, safe, heating and luggage storage. For recreation there is a tennis court. It has 123 rooms available.