Entertainment walls, closets, window seats, something useful where there was only space before: built-ins. Adding these things is the mark of a very good finish carpenter/cabinet maker. I have done several of these in my career. Older dwellings add particular problems. Nothing plumb or level for example. My particular skill is adding built-ins to make them appear level and plumb when the space does not allow it.
Making built-ins usually involves making the main pieces in my shop and then adding them to the existing space followed by mouldings or other trim and finish. I work very hard at the working relationship between my client and me, providing almost as much work in design as in actual installation.
A lot of times, the built-in can be constructed from scratch within the
existing area: window seats, for example.
I don't declare the job finished until the client completely agrees that they got what they wanted.
Nothing pleases me more than these types of jobs.