I have been inspired by a couple of recent articles by Chris Sims over at Critical Hits, which discuss using minions to spice up encounters.
"Minions are Spice":http://critical-hits.com/2010/07/22/minions-are-spice/
"Minions of Differing Flavors":http://critical-hits.com/2010/08/19/minions-of-differing-flavors/
While Chris discusses these spicy minions in a 4e context, I was inspired to spice up the encounters in my own 3.5e game in a similar fashion. For me, the spicing-up has mostly taken the form of playing around with a creature's standard tactics, description or other rules in order to make things more interesting
Here are some specifics I have worked in to my game: During my last session, my party battled an infestation of mimics at an arcane library. If caught in melee, the mimics would use their standard grappling attacks as described in the MM. However, at range, they took to throwing books at any intruders. Furthermore, I included an advanced huge-sized mimic, which would pick up anyone who engaged it in melee, and throw them at their companions the next round. This amusing tactic played out with a summoned giant mantis to wonderful effect.
Then today, I created a special little table in preparation for some vampire hunting my players are setting out to do. I have decided that, similar to in the movie, The Lost Boys, vampires in my campaign do not go out quietly, or in the same fashion every time. I came up with a table for vampiric death-throw effects that will accompany their burst into gaseous form. They can do anything from dissolve into a pile of ash, to bursting into a swarm of flies, to coating their destructor in a fountain of blood requiring a save vs. being sickened.
I am interested to hear how others have bent the rules, reskinned or tweaked creatures and settings in order to spice up their encounters.
P.S. I also find it amusing that I took this break to write about spice while I have been sitting around reading Dune.