Pretty quickly I was able to put together a very simple GUI application for converting between Celsius and Fahrenheit. This was a first day exercise when I taught five-day NeXTStep bootcamps back in '93-'94.
Of course, some advancements have been made such as alignment guides within InterfaceBuilder. There are probably many more, but I didn't find them in my 30 minutes of play time. (I also had to get used to a one button mouse.)