If you want to enjoy some hours in a lovely green ,blooming and smelling area you should visit the "Botanischer Garten der Ruhr Universität Bochum"
Although it belongs to the university it´s open for public visit at special hours.
There are small theme beets and areas, with many foreign and domestic and endemic plants, most plants marked with name and description. It is located within an area with old trees and little ponds and sometimes you are feeling like walking across a forest.
I like the "Chinesischer Garten" very much, it´s that romantic (some people say kitchy:-)).
There are some greenhouses and changing exhibitions and events, actually you can visit Europe biggest exhibition of flavour, scents from heavenly to bestial.