I live in Heidelberg, and so does Lindsey. It takes 5-6 hours to Duisberg if we take a regional train, which is cheaper as an IC/ICE train. An IC/ICE takes only 2½ hour, but costs 55-74 EUR (I have a discount Bahncard, Lindsey does not) each way. On a Saturday or a Sunday it is possible to buy a Weekend ticket for 30 EUR that will take up to 5 people all over Germany, but only in the regional trains. I could ask Lindsey if she would like to travel low budget with me to Duisburg, but seeing as she is pregnant she might not like the idea of getting up really early to go sit in a train for several hours. I will ask her next time I talk with her.
