In terms of its space, not looking like a blue police box...
Background: I'm going away to a 2 day conference (2 nights away) and on the last day I will need to go out to a bar in the evening. I will not have anywhere to put my luggage so am planning on an extreme capsule approach and am hunting for a bag that is big enough to hold everything but small enough to keep with me on the final evening.
So in summary: smart-ish, not too huge but with tardis properties. Does anyone have any ideas?