Markeaton Park is a large open area on the outskirts of Derby, accessible from the A38 / A52 roundabout.
The park has a large lake and lots of facilities are normally open such as activity areas for children and a craft village with a cafe.
Unfortunately none of that is open at the moment but it’s still a great area for wandering around for your exercise.
When we visited between 9:30 and 11am it was not crowded and it was easy walking around. In busier times there is lots of open space to play and picnic, etc.