68. Orcs that act with intelligence and forethought. I nearly wiped out my 5th-level party with a few orcs that planned ahead. Once the patrol fired off their thunderstones as alarms to alert the rest of the nearby orcs, things got ugly.
69. Make good use of weird architecture (Stronghold Builder's Guidebook has some great examples). Again, in the Friday night session, the party was exploring the House of Stone, which had been deserted for centuries (and, at least the ground level, had been claimed by the aforementioned orcs), when they came across a room full of completely preserved meat (near the kitchen; this is a wondrous architecture item from the SBG). Especially once the wizard cast Detect Magic and found out the entire room was, essentially, magical, they freaked out.
70. Hint at something big and very nasty lurking outside of the PCs' line of sight. Once again, Friday night (wow, that was a good PC-freaking session), the PCs threw something down Stoneturn Well, and got back the sound of some massive beast growling. Now they won't go down to the lower levels.
71. Have a dragon use it's breath weapon to drop a ceiling on the PCs. This is amazingly effective.
72. Monsters with class levels. In our old campaign, at around character level 10, the PCs came across a room with a goblin in it. They said, almost in unison, "Just one goblin?!?!?!" A disintigrate, mage armor and a couple of fingers of death later and they got the clue. Also, remember, a kobold barbarian makes use of d12 hit dice. And 2-3 levels of fighter added to an orc make previously cannon fodder baddies nasty for lower-level parties.