getters and setters very often don't come in pairs

To help in applying what you've read here to situations that aren't specifically covered, think about whether something is conceptually a property. If you've got a bunch of operate_on_Doohickey type methods, regardless of whether any of them happen to be get_ or set_, you probably have in your model the concept of a Doohickey property.

If you've got only one operate_on_Doohickey method, there's still probably a Doohickey object in your model, but it's not interesting enough to implement as an object. Of course, you might expand your functionality in the future, in which case you might end up wishing you had coded it up as an object.

I definitely wouldn't code up an object for a read-only property. If Area wasn't settable, then my Circle object might have an Area method that simply returned the area of the Circle. Note that it wouldn't be getArea. But if it had numerous possible output formats, I'd have it create a Measurement object (not a sub-object, but one that exists outside of the Circle class hierarchy, because it's not specific to the domain of Circles) so I could get Area->inSqFeet or whatever.

The design decisions are yours, of course.