I've actually been to Gdynia and Sopot (ironically on a cruise ship long full day excursion from Gdansk). We had a very good local guide, and an excursion that was crazy cheap as it wasn't on one of the International cruise lines.
While Rick's description is in many ways quite accurate there is more there than meets the eye. Gdynia does have a fair bit of history (and a lot of WW2 history especially), although the majority is indeed Modernist Architecture. However that Modernist Architecture has resulted in it becoming a UNESCO World Heritage Site. Which just goes to prove that one man's meat is another man's poison.
In Gdansk get beyond the tourist parts and a lot of the city is modern, and I mean modern, not modernist in the style of Gdynia.
Few people would know that Gdansk is a cruise port because both the cruise and adjacent ferry ports are well outside the city centre, linked best by the water bus service (although buses and trams go down to both sides of the river, and there are touristy things down there, which you wouldn't know about until sailing in.
I've always thought that if I were to go back (not on a cruise, maybe on the ferry from Sweden) i would prefer Gdynia/Sopot to Gdansk, partly because they are not international tourist central- rather more domestic tourists.
In the context of your trip, I'm unsure whether they are the best use of your last day, probably Gdansk/Malbork may be best. However it could end up as an 'if only', so I'm not saying don't go.
I think the few hours I was there, the historic centre of Gdansk was just packed with tourists, and I didn't really gel with it, but that is very much me. Maybe my expectations were set too high.