This pretty reservoir in the centre of Birmingham has a nice circular walking trail running around the perimeter. There's a good sized car park at the south eastern end of the water. You can pick up the well surfaced trail from here. It runs for just over a mile and a half with lovely views across the water and some woodland and grassland areas. You can walk along the dam and enjoy great views of the cityscape of Birmingham. The site is also a local nature reserve so look out for a wide variety of birdlife on the water.
If you'd like to extend your walk you can pick up the Harborne Walkway in the adjacent Summerfield Park. The nice surfaced path runs along a disused railway line from Summerfield Park to Harborne. You are also very close to the Worcester and Birmingham Canal so you could pick this up and enjoy a waterside walk or cycle.