When you hear people talk about shopping in the San Lorenzo Market, there are actually two markets they may be referring to. One is the popular-with-tourists outdoor market full of leather goods as well as Tuscan and Florentine souvenirs. The other is right next to the street market, but it's indoors with food vendors selling everything from meat and fish to vegetables and bread. Both are worth visiting.
The indoor San Lorenzo Market - more commonly known as the Mercato Centrale di San Lorenzo - has two floors of food stalls, and it's an excellent place to stroll through if you're wondering what ingredients are fresh and seasonal (and therefore what you should look for on menus) or picking up supplies for a picnic or the kitchen in your rented apartment.
Some of the deliciousness can be brought home as souvenirs, too, although be sure you know your country's laws regarding bringing meats and cheeses back home before you spend the money on something that may get confiscated. Many cooking classes in Florence include a walk through the Mercato Centrale at the beginning so visitors can learn about Italian food markets and pick up the ingredients they'll be working with.
The outdoor San Lorenzo Market is the one that's most visible, since it's outside and open in any weather. The streets around the Basilica di San Lorenzo crowd with stalls every day. Most of the vendors are selling leather goods - everything from beautiful leather jackets to belts and wallets and purses - while others sell other sorts of souvenirs. This part of Tuscany is known for its leather, so you can sometimes find really great deals in markets like these.
You should be aware, however, that the stalls are operated by people who own actual shops nearby, so if you're looking at a jacket or anything else that has different sizes, you'll eventually be led from the stall to the shop. That's where the hard bargaining (including the attempts at the upsell) begins. Bring your haggling skills, be prepared to walk away if the price isn't right, and be very careful with your valuables. The outdoor market - like any busy outdoor market - can be a boon for pickpockets.
