I still didn't decide what to make exactly -- although I do have some vague ideas. In the mean time, I'm playing with Tiled and trying to reproduce the visuals similar to those in Portal's tutorials and ads:

Sorry for teasing everybody with the graphics. After a week of absolutely no time to work on the game, I'm pretty sure I won't make anything bearable. I still plan to work on it on Saturday if time allows, but I don't think I will be able to make in one day anything that would be acceptable. Sorry, next time I will try and take time off work.