I'm still writing up the technical stuff (still drafting it out) but basically, in your map the player will be limited to the bounds no bigger than X (somewhat small) enclosed or not, but playable space should be cut off somehow (invisible walls, convenient debris). Then there needs to be a view of some sort to look out upon. The view needs to be separate from the playable space; meaning, (like Aaron said), the player should not be able to reach that view. However you want to pull that off (i.e. 3D skybox, custom textures, a mix of whatever, anything goes as long as it can be run in real time within the Source game of your choice).
Other things to think about: Your playable space itself (whatever it may be) may be allowed to move (example: a gondola moving down a mountain side, or a vehicle moving through a city block). You might get a better chance of winning if you have animated scenes rather than just static scenes. Remember A vista scene in a game should incorporate sounds too!
Almost done with the draft; I might have something ready by the end of this week!