Go to the beach (of course!): Let's be honest, the main reason you probably are going to Torredembarra is for the beach. The three beaches here are Els Muntanyas, La Paella, and the beach of the Maritime district. Els Muntanyas is a beach that you definitely should go to if you have time. It is very pretty - and rare, because it is one of the few natural beaches left on the Catalunya coast and is a great place to bring your camera and take pictures. La Paella and the beach of the Maritime district are the most touristic beaches and are the beaches you will probably be laying out in the sun with your family or friends.
Walk around the city center: The center of the city is a nice place to walk around. There are some pretty plazas, a lot of cafes, and some good boutiques and stores where you can go shopping. Also, keep in mind that the center really is not so big.