Definitely try to have a day out on the 218s towards Westerland (assuming you're interested in such things!). But bear in mind that the 218-hauled trains (4 each way) are all IC, so aren't valid on the Hamburg regional ticket (RE trains only), so you'd need to buy an appropriate ticket. Others more familiar with the area may be able to advise as to what the best ticket is. The RE trains on the line (approximately hourly I think?) are hauled by diesel TRAXX locomotives.
There are one or two other 218-hauled trains I believe, but I don't know what they are. Again, someone more familiar may be able to advise.
If you're interested in freight, there's lots! Harburg station is good for seeing it pass through. If you fancy a trip out of the city, Maschen marshalling yard (the biggest in Europe?) is to the south. No idea how best to get there though, nor how easy it is to see anything in the yard.