With my regular walking places being Ferry Meadows (Nene Park) and Bourne Woods, I sometimes forget I have Deeping Lakes Nature Reserve practically on my doorstep. 180 acres of flooded gravel pits is not easily missed; but it is primarily a nature reserve, and only a narrow part of it is open to the public.
Having said that, it has good parking, and a pleasant walking route along the south bank of the main lake, and then looping back along the River Welland. The walk itself is only about a mile long, but there is plenty to see, with benches and bird hides along the way.