Not too far away, near Hala Targowa is the U7 bowling alley. There’s a particularly sordid story behind this one; after the war, German prisoners seconded into rubble clearance noticed a horrific stench emanating from the ground. When they broke through the entrances, which had collapsed under bombardment, they found it literally flooded with gunk. The air-raid shelter, built below the water table, had flooded during shelling, and those trapped inside had been slowly drowned in the most agonising way imaginable. The putrid soup in front of them was decomposing remains mixed with stagnant floodwater.
Elsewhere, check out the Napoleonic hilltop forts on Gradowa Hill, up the hill from Gdańsk Główny. Built 46 metres above sea level the views across the Gdańsk are awesome, and their strategic importance wasn’t lost on the Germans. Close up investigation is obstructed by fences and warning signs, though you can just about make your way up a concrete observation post installed at some stage during the war. If you're taking the kids to Centrum Hevelianum why not take a look?
Lastly, but not at least, the forests of Gdynia Redłowo (P-4, P-5, P-6) are abundant with small concrete bunkers as well as a whole bunch of small trench networks.