The Rastro is a must-do in Madrid. It is a huge flea market held every Sunday from 8 am to about 2 pm. It is in Old Madrid near the Latino metro stop. You can get anything at Rastro - scarves, earrings, sunglasses, shirts, Levi's, shoes and leather goods, books, household items and even army- issued goods, among a myriad of other things. You must be very careful to watch out for pickpockets! El Rastro is notorious for this type of petty theft. Do NOT carry large amounts of cash or any valuables on you when you go to el Rastro.
Grocery stores are a bit different in Spain. You will notice that though they can be like local grocery stores in the U.S., the variety of choice is different. For example, in the U.S. there are numerous brands and styles of canned tomatoes, whereas in Spain, you may notice that there are only two brand and three styles of canned tomatoes. While shopping for fruit in Spain, only touch the fruit with the plastic gloves provided. Do not handle the fruit with your hands. You may not always be able to use a credit card at the grocery store if you are not spending the minimum amount. Be prepared to have cash and in small denominations. And don’t forget, grocery stores are almost always closed on Sundays. Some common grocery stores are Mercadona, Dia, and Carrefour. Undoubtedly you will find your own store close to you.
You will find that Madrid has some of the best clothing shops in Spain. The variety is tremendous. You can find your designer brands, chains, and boutiques across the city. Ask your host family about the various shopping districts in Madrid.