I will be in Dusseldorf for 6 nights in October, attending a trade fair in Cologne. Will have 2 colleagues also.

In between my stay, I would like to make a day trip, maybe even beyond Germany. Don't mind a train trip. In 2007, I when I went to Frankfurt, I took an ICE train to Berlin for the day. 4 hours each way but in Berlin, I was able to do a tour, lunch.

Any suggestions?