ress up your walls
Put colour and texture to optimal use. The living room can have a beige tone on the walls, while the dining area could be a terracotta tone. Niches can be highlighted in bright colours like flaming reds and bold blues, and the kitchen can have yellow to bring in some warmth.
You can experiment with a lot of colour depending on the amount of natural light entering a room, the size of the room and of course the colour of the furniture.
Once you have worked out the colour dynamics and texture applications (when paint is used in a textured effect that creates patterns on the wall), you need to highlight them by paying attention to what goes on those walls.
Good art is the best investment. Failing that, photos framed in a series reveal your world -- family, friends, travel, accomplishments -- all there on the wall.
The retro look could be brought in using poster art and artifacts, like say an old gramophone, a nineteenth century clock etc.
You could also pick up some interesting digital art which always intrigues visitors. Digital art is much cheaper than buying a proper painting, but a lot better than a poster.
The shelves used for books, curios and other accessories allow for a lot of experimentation. Use different kinds of shelves in different rooms, to co-ordinate with the room's décor.