I finally chose one of Steven Reddy's courses. He does a lot of whimsical sketches of interiors - cafes, shops, cluttered spaces - and in the course he shows how he does it. The great thing about his technique is that you draw whole scenes, not just single objects, without giving much thought about perspective - or straight lines. And still you get a great representation of what is in front of you. The aim is not to end up with an accurate image, but with more like a subjective view of your surroundings. You include what interests you and leave out what doesn't. You grant yourself lots of artistic freedom. The resulting drawing is mainly for yourself.
But I'll show you my first attempt anyway:
