Park in the parking area near Bellevue Pond. There are multiple entrances to the reservation (some closer to the reservoir itself than others). I have spent days in there hiking around and never seeing the water and there are days when I just hiked right in to the water and sat and enjoyed it. The trails are perfect for mountain biking as well. I spent so much time here when I lived in Boston because it felt like you had completely escaped the city. You can hike around several miles and never see another person in the cooler months. You can find a quiet spot by the water that is completely your own. And since the reservoir is man made, the water is completely clear and still. It feels like a pristine lake. Middlesex Fells is definitely one of my favorite places. 

See trail map here for Southern Loop. If you want to do this one in the winter, bring your snowshoes!