Running the twin beds lengthwise against the walls, with headboards against the corners, usually will allow for the most floor space, but the trade off is that it is harder to make the beds that way.

Can you give us a diagram of the floor plan, showing door and window placement? We can't tell you where to place the window except to say on an outside wall, right?

I'm no pro, but I'm curious-does this apartment exist? It reminds me of a project my husband took in an elective class, Intro to architecture, where he was required to design a living space as part of his final grade (Or is it some sort of student housing??) I'm asking because 1) All of the bedrooms are the same size and are isolated from each other, 2) the dining area at 10 x10 in the middle is going to be a corridor and you'll have almost no space for a table and chairs. 3) Then also all the bathrooms are ONLY accessed through the closets, which is extremely awkward for visitors. As far as the bedrooms, not necessarily an exciting choice, but your easiest choice is a bed against the toilet wall, with at least one nightstand, and a dresser on the wall by the door.

Why not paint it in a pastel colour? Add a carved chest that doubles up as shoe rack and organiser to store your shoes below and hold keys and bills in top drawers. You could hang a painting or a statement mirror above the chest and highlight with wall sconces or beautiful table lamps to shine a soft light around it.