Sunday activities for busy professionals? (Cologne)

While I would love to participate in events during the week, a nice meal out and all sorts, I just don't have the time. I commute two hours every day and by the time I get back into town I just want to jump into my sweats and chill :)

Are there any activities slated for Sunday afternoons? A visit to the zoo? A nice lunch somewhere? A museum visit? Anything?

I have to admit I'm a very relaxed person and prefer to not have to dress up to go places :)

Gina