One of the things that was always hardest for me to grasp as a DM/GM was exactly how monsters approach combat. It never seemed like a group of orcs should stand there fighting a straight-up opponent in any sort of organized way, whereas a hobgoblin squad would be very organized and use standard battle tactics. Sure, that's what the book says to do, but how many players and storytellers alike actually have military training to the point of understanding or planning a combat situation? While I have never served in the armed forces, I have spent time in ROTC, CAP and participated in a number of "coordinated exercises" lead by members of the military. As well as having read many books on the tactics used by Rommel, MacArthur, Patton, Longstreet, Grant, Lee, Sun Tzu, etc.
So I figured I'd do a little blurb of how various, imho under-used monsters could approach an encounter. Going in roughly alphabetical order, the first is Animated Objects.
These little annoyances come in all shapes and sizes, materials and mutations. It's easy enough to figure out how a knife, sword, candlestick or hammer would work but what about an ottoman or foot stool, birdcage or chained tapestry. The first thing to figure out is what exactly is animated and why it is where it is.
I always liked putting these in a study or library, waiting till the PCs decided to sleep or are otherwise distracted and then activating the objects at random times. There's nothing saying that the object has to respond immediately to an intruder. So, give the players a nice, soft lounge chair to rest for the "night", then after they have settled in and only 1 or 2 are on watch (or none if they are that *daring*) have said chair begin suffocating the PC trying to rest. What about that empty birdcage in the corner, you know, the one hanging from 10 or 15 feet of chain. Well that's just a trip, entangle or disarm waiting to happen. Or better yet, combine them to use the chain to constrict around the PC resting on the couch that is trying to suffocate him.
I also like to add "flavor" to some objects - spikes to a chain, sharp edges to a table/desk, burning pitch to a brazier. Most of the time these things just do a bit more damage, but consider the vision impairing effects caused by a flying tapestry or how much steam a cauldron could make before it attacks. Remember these objects aren't intelligent, but whoever created them probably was and putting the objects into position to be as effective as possible is something they probably considered.
One of the other things I like to add to animated objects is rust, rot or disease. Think about it, that object may have been sitting there for hundreds of years and dungeons are notoriously damp. When was the last time an adventurer took any precautions after getting out of a dungeon and had a cure or remove disease spell cast on them?
The other place I really like to use animated objects is in conjunction with traps. Nice, you found the arrow trap and used that candlestick to mark the location of the trigger in case you need to come back this way in a hurry. Sucks that it moved a few feet from where you originally sat it down. Oh, your fighter didn't find the pit trap and fell 30' and all those spikes/swords/spears at the bottom of the pit were animated. I have used this combo once and the players really didn't like it, but while they were working to pull the unfortunate fallen soul up with a rope, a pair of animated sheers came out from a side room (weaving room) - he fell again...