"Is there anything must see?"
As much as possible

Will you have time to travel on it? I'd say a trip to Drei Annen Hohne and up the Brocken would be the best quick fix. If a little time at DAH there are some multi train crosses there to watch. Another trip would be via the mainline to Quedlingburg to do part of the Selketal line as it's faster if time is limited than going all the way round the HSB.
Look for Nemos & the lads threads on trips there as there are some good photos to tempt you
Links to their reports in this thread
https://www.gscalecentral.net/g-scale-travel/hsb-trip-what-is-the-must-to-see/