This is such a wide field that it would really depend on what sort of mechanical engineer you are looking at. For instance a design mechanical engineer might spend the day actually calculating heat losses and how to overcome them in a building (heating and ventilation engineering) or maybe designing all the different components of a valve assembly - from concept through to manufacture (boiler and steam plant engineering). If you were to be based on site, then you might be overseeing the installation of equipment or the maintenance of the same. Mechanical engineers are also to be found in the aviation industry, in the construction field, in public structures (say the installation of a new power station), in the general manufacturing industry (someone has to design all those must have gadgets like tv's, mobile phones, dvd players, washing machines etc) and in the food industry - production lines etc.
I think the only common thread to all of these different jobs is that they mean you will have to use your brain and be quite creative and practical. If you are good at taking things apart, putting them back together and understanding how they work, then this might well be a good career for you !