Schwerin is a huge area and I have "only" been to the massive castle, it's gardens and the entire lake next to it. The castle costs entrance and unsure it's worth the costs, not that it was too expensive and kids are free, but since you are already here and the entire day will pass easily by spending hours in the castle alone you might as well go for it. I think the scope of the site has changed over the years too. At first it seemed to only be the castle, then add the lake around it, and finally it contains an entire area of the residence. I feel it's still enough to just go to the castle and walk around it and see the major parts of the site though.

If for some reason you don't want to see the inside of the castle, which is understandable if you saw many others, then the park will keep you busy for a while too. Just walking to the Lake Schwerin for the castle takes a while and then you walk around, see the picture some take from far away, and eventually you are back in maybe 1-2 hours easily. The main attraction IS the castle and I have to say it's beautiful to look at so if you are standing across then the images are better. Pictures online that have a drone go high are even better. The castle takes almost the entirety of the island which is a great spot for the castle.

Back in 1857 Frederick Francis II had the building converted and cemented as a romantic spot, well placed in the historicism in Europe. Petermännchen, a small household spirit who is said to live in the Schwerin Palace is also "present" but you'll have to make the connection yourself. Obviously nobody will believe the ghost stories nowadays (or maybe they do?) so it's more of a joke for visitors.

Expect to spend the entire day here.