For your visit you need a full day to be able to explore city’s major sights while keeping pleasantly unhurried pace.
Distances are walkable in all cases.
Worthy attractions: Palazzo Pubblico and Torre del Mangia; Duomo; Basilica di San Domenico; Museo dell’Opera Metropolitana.
Left for another visit: Pinacoteca.
Last visit: August 2013.

Siena, often overshadowed by its erstwhile rival Florence, is a town that easily awes its visitors.
The medieval center of the town, where automobile traffic is limited, is bigger than any other in Tuscany, while remaining relatively compact and walkable (there are some 30° climbs along certain side streets, though). The concentration of magnificent palaces dating to 11-13th centuries is probably even greater than in Florence.
The heart of the town, Piazza del Campo ♥♥♥, is one of those places which never fail to elicit a Wow!, no matter how many times you see it. Laid out in the shape of a giant shell, it is the setting of Siena’s famous horse race/festival Palio delle Contrade, and its entire expanse is often full of lounging people. Cafes, restaurants and fine medieval palazzi line the Campo’s fringes, dominated by Palazzo Pubblico ♥, the graceful Gothic town hall. Its former magistrate apartments offer a dozen of grand rooms with frescoes in different states of preservation, and some additional exhibits in a couple of rooms. The palace also allows access to Torre del Mangia ♥, the second highest medieval tower ever built in Italy. The view from the top is pretty good, albeit through wire mesh, but a word of caution: Access to the tower is regulated for maximum allowed number of visitors at any given time; on one of our visits, during the supposed off-season, we chanced into a 25-minute wait before having been let in. The climb is not very hard, since each flight of steps is reasonably short, but the steps are extremely narrow, and two people going in opposite directions cannot pass without brushing against each other.
Do not let the imposing Palazzo’s ensemble to make you miss the little Fonte Gaia on the piazza’s northern edge. In addition to exquisite reliefs, the fountain has a distinction of getting its drinkable water from a 500-year-old aqueduct. You can fill your water bottle from a valve.
Siena’s Duomo ♥♥♥ is one of Italy’s grandest cathedrals, a spectacular mixture of sculpture, painting and Romanesque-Gothic architecture. Among its treasures are masterpieces by Donatello and Michelangelo and a fine inlaid marble pavement composed of 56 panels, depicting mythology and the scenes from the Old Testament. Do not miss Libreria Piccolomini with its magnificent fresco cycle. Certainly go inside the cathedral; but just standing in front of its entrance, looking at the intricately ornamental façade, is a treat. Many of façade statues are copies, with originals on display in the Museo dell’Opera. The museum holds several floors of iconography and religious statuary, including works by famous Sienese, Duccio. Access to the roof with panoramic views may require a significant wait not unlike that at Torre Grossa, since the platform can only hold 25-30 people, with a new group let in every 15 minutes.
Note: You can buy your tickets for the cathedral visit by the baptistry, which has negligible lines compared to those by the cathedral entrance; you can then walk straight to the entrance, bypassing the long line.
Opposite the cathedral on Piazza del Duomo is Antico Ospedale di Santa Maria della Scala ♥. We walked in only once in our several visits to Siena, but the grand Sala del Pellegrinaio is worth lingering in for a while.
Basilica di San Domenico ♥ is spacious and airy, devoted largely to St Catherine of Siena. On the way between the grand church and the city center, we once visited Santuario di Santa Caterina ♥, a beautiful small church and no less beautiful separate chapel, certainly worth a look if you are passing by.
There are several other churches and palaces that may be worth a visit, but we content ourselves with a leisurely walk along the narrow streets. Some of the palaces open the doors to its courtyards, gratis, and some hold exhibitions or allow tours. We especially like the tranquil courtyard of Palazzo Chigi-Saracini, graced by an old well.
Occasional passing cars are a nuisance, and tourists tend to flood major routes, but there are uncounted majestic architectural sights and picture spots, complemented by fashion and food shops.
Car parking
There is a number of car parks within walking distance of the city center (although, it will take you at least 15 minutes to get to Piazza del Campo from any of the lots). If I am able to find a spot at Fortezza, which is free, I park there; otherwise, the stadium parking is an option.
Places to Eat
Our culinary exploits in Siena have mostly been limited to pizza, gelato and coffee on and around Piazza del Campo on various occasions. The pizza has been delicious on all occasions, and it is a fun little experience to buy a box of various slices, fill your water bottle by the Fonte Gaia and sit down somewhere on the piazza’s stones for an impromptu meal.
On our very last visit, at the advice of our eldest, who actually had spent several weeks in Siena studying Italian one summer, we had a full lunch at an out-of-the-way unglamorous pizzeria/spaghetteria. We had few complaints, and the bill was about half of what we normally spend. Don’t be afraid to experiment with no-name places when in Italy.
Monteriggioni
Monteriggioni is a tiny walled-up hill town. Although some guidebooks recommend it as a top attraction, I can only see it as a brief drive-by stop when around Siena. The town walls can be ascended in two places, but the surrounding vistas are only marginally impressive. The only square is charming with a tiny Romanesque-Gothic church.