Cape Cornwall is part of the West Penwith International Dark Sky Park. The car park does have a slope, but it allows you to drive to a spot where you can see the stars over the ocean. There is a wonderful little café and accessible toilet facilities (usually open seven days a week from 10.30-16.30). Visit the National Trust website for more information.
For somewhere a little easier to access, head to Marazion to see the stars above St Michael’s Mount. There is an accessible walkway along the seafront and good facilities nearby. As an Access Cornwall reviewer notes, “I love to come here to Marazion and just lie on the wall by the sea and spend time looking up at the stars.”