I'd say that The thing to visit would be Sagittarius A*, because it's the most unique object in the galaxy. There's only one supermassive black hole.
Other than that, it's also worth going to the galactic edge somewhere, and the galactic roof / cellar (as far above / below the core you can go) too. Since these aren't specific systems, but areas instead, you can actually explore for undiscovered stuff at these.
As for already-discovered stuff, there are quite a lot of destinations to see if you want to do some sightseeing. There are tourist beacons in-game, and if you don't want to look for the missions,
here is a list. Beyond those, the
Galactic Mapping Project can give you an even better list of things to see.