Bergpark Wilhelmshöhe
Worth visiting is the "bergpark" mountainpark which is supposedly the largest in europe and second largest park on a mountain slope in the world. the park contains the herkules statue at its top end which is towering over the city and has become its symbol. further the park contains loads of smaller features which are mainly fake greek temples, fake medival castles, fake querries - you see where we are getting ... not fake though are the rembrandts and rubens paintings in the schloss in the park - an entrance fee is charged. the park also features during summer months the wasserspiele watergames every wednesday and sunday starting from 2:30pm. various of the features such as the cascades below the herkules statue, the fake waterfall and the fake roman aqueduct have their water supplies opened for half an hour or so each and the crowd of tourists will move from feature to feature starting on the higher parts moving downslope.
Centre
The Centre of Kassel itself has been thoroughy destroyed during WW2 and was rebuilt in a post-war fashion - ugly. Hence, the inner city is mainly dominated by bank and mall buildings of the modern sort. However, in walking distance from the main high street, you can reach the Karlsaue, another, french style park with some old buildings containing all sort of museums tapstries to astrology. That area is also the usual place for the Documenta art exhibition to happen every five years.
Getting a little bit out of the city can be a good thing. Near Lohfelden there are beautiful forests and ponds that have really good trails. Also just past there are some great churches and markets.