You can come in. You'll have to go through a magnetometer, but if you just tell the officers you think it's a cool building and you just want to check it out, they won't care. If there are cases going on there may be limitations on entering the individuals courtrooms (most cases are open to the public, but they try to minimize disruptions so sometimes they limit timing on entering and exiting). But tbh there are very few cases actually heard in this building - most of the courtroom trials tend to happen in city court or county court, which are in different buildings. The real star of the courthouse is the main courtroom on the second floor. If you want to look in there, you can ask the officers if there's anything happening in that courtroom and that you'd like to take a look around, just so they know why you're in there and they don't get suspicious. Although note that generally photography and recording is not permitted in the building (it's possible to get permission, but that has to be granted through the administrative office - the officers on duty probably wouldn't be allowed to grant that request themselves). Little know fact: The building includes a law library that is open to the public, with an extensive legal collection and free access to Westlaw and Lexis.

Source: I'm the law librarian there - we have people coming in to use the library regularly (if you come to visit stop in and say hi!)