There is unrestricted access to Sanssouci Park during daylight hours. You can easily stroll around there for two to three (or even more) hours. Access to Sanssouci Palace is limited, you can only see it with a guided tour. The tour takes approx 45 minutes, queing for the tickets may probably take longer and even be unsuccessful, there is a max number of 2000 visitors per day.
Film Museum is worth it if youre interested in those things. But Potsdam has more to offer:
Glienicker Brücke* (mind your trenchcoat), where during the cols-war-times spies were exchanged between east and west;
Alexandrowka: Russian-style block houses from early c19 plus orthodox chapel on the hill. One hill further the architectural folly of the Belvedere am Pfingstberg*, great views.
Schloss Cäcilienhof*, where the 4-powers-agreement on Germany after WWII was negotiated. The villas, where Stalin, Truman and Churchill stayed, can also be visited.
Dutch Quarter* in the city center from c18, for the Amsterdam-feeling.
You can easily spend more than one day in Potsdam, since this list is still incomplete.