Despite some characters’ appearance to the contrary, uniforms can’t stay on forever. At some point, they’ve got to come off, which means, generally, that something else goes on. Some will, of course, value the freedom their off hours permit to express themselves through their attire, or at least enjoy the comfort of less formal clothing. Others prefer the simplistic- why go to such lengths when a t-shirt and jeans would be more efficient? Some my prefer garb of a religious or cultural nature- Vulcans appear to enjoy an affinity for flowing robes and the like.
What does your character like to wear off-duty?