There are good reasons why these two Hanseatic towns were inscribed together as one World Heritage Site. They have a lot in common: a similar medieval layout, churches in Brick Gothic architecture as their main landmarks, well-preserved gabled houses dating from the time of the Hanseatic League and traces from the period of Swedish rule.
In 2002, when Stralsund and Wismar were inscribed, the Hanseatic League was already well represented on the WH list. And even Germany had a similar WHS with Lübeck, inscribed as early as 1987. So, nothing outstanding about Stralsund and Wismar? Indeed, if you have already visited other Hanseatic towns, Riga for example, you may feel that this WHS has nothing to offer that you have not seen elsewhere, and probably even better. Lübeck has the Holstentor, but neither Stralsund nor Wismar have such a distinctive and unique building. On a positive note: the medieval townscape has largely been preserved in both cities, there are hardly any ugly modern additions. Again, Lübeck as a counterexample.

The old town of Stralsund is located on an island. From the tower of St. Mary's Church, you have a beautiful view of the historic town center, the sea and the island of Rügen.The best moment in Stralsund was a coffee in the sun on the Old Market Square with a view to the façade of the Town Hall and the towers of the St. Nikolas’ Church, and the gabled houses in my back.

In Wismar there is also a St. Mary's Church with a viewing platform (actually only the tower has been preserved). You don't have to climb stairs, there is an elevator, more comfortable than in Stralsund, but the view is less impressive and I would not recommend it. In Wismar, I liked best the area around the St. Nikolas’ Church (even the churches have the same name) and the Old Harbor (photo) with the Wassertor, the gate from the harbour to the town centre and the only preserved city gate in Wismar.
Cineastes will certainly remember the Wassertor as the location of a scene from the silent film Nosferatu: A Symphony of Horror by F.W. Murnau from 1922. It is perhaps the most impressive scene from the Wismar setting: The vampire Count Orlok walks through the Wassertor with his coffin, in which he sleeps during the day.

I visited Stralsund and Wismar in August 2020 and spent half a day and a night in each of the two cities, which is enough time for a proper visit of this WHS. The history of the two cities ran parallel, from the period of the Hanseatic League to the recent times of the GDR and German reunification. Therefore, they share a lot of similarities, like twins, although they are about 150 kilometers apart from each other (Lübeck is much closer to Wismar).
I really enjoyed strolling through the old streets, exploring the architecture, the cafés and shops. It can be very relaxing to visit a city without paying attention to opening times or queuing for tickets.