How about vistas? A few locations in the map where you can see special stuff? Could be a reward for finding certain BTS areas, but they can also simply be part of the map itself.
Personally, I hate time limits. A few relaxed limits keep the tension up, but four second-tighted timers running simutaneously? I get a frikking headache of the tension... (I am not referring to any of your maps, just Portal maps in general)
On the opposite, actually timed stuff which can be lethal but with a very easy solution. Great tension builders (I am refering to the Fire Pit at the end of C19 in the original Portal. Escaping was very easy, but the actual faceoff with a certain death in a limited time made the part very exciting. Another example is the actual battle with GLaDOS.)
Maybe you can try the Cubeism-idea: various cubes with varying characteristics, for example a lighter one or one that's on fire.
Some kind of boss battle? A mini boss, not GLaDOS or a giant mecha or anything, but something you have to fight as part of a test. For example a bunch of Rocket turrets connected to a large, central pillar that sinks deeper into the floor with each surrounding Orb Catcher that is filled. It is your job to guide the Orbs into the Catchers whilst being the constant target of rockets.
Large ammounts of Androids stuffed into a way too small chamber do not count as a miniboss.
Those were just a few small ideas...