Try looking at Binder in Tome of Magic. All his stuff is Su and either always on or at will spammable (sometimes with a 5 round recharge time between uses). He has a few vestiges that give debuffing or damaging auras, Like Focalor or Geyron.
If "doing nothing" can include some sort of performance (mime?), there are Bard variants that get debuffing performances instead of inspire courage and such.
to do nothing but actually being quite active, you could do a Changeling Mindspy Cabinet Trickster build. The annoying thing is due to each PrC's pre-reqs for min. level entry and the need to max out both (they're short PrC's, though) to really get the useful stuff... you won't actually finish the combo until very high level.
Here's an incomplete build I had for such a character, just a thought experiment. I'm sure the feats could be better ordered (like sacred vow and vow of nonviolence coming closer together O_O):
Changeling [Humanoid (Human, Shapeshifter)]
CG Martial Rogue 4 / Factotum 3 / Marshal 1 / Cabinet Trickster 5 / Mindspy 5 / Warshaper 2
Progression: rogue 1, facto 1, marsh 1, facto +2, rogue +1, CT 1, MS 5, CT +4, warshaper 2, rogue +2
Str 8, Dex 14, Con 14, Int 16, Wis 8, Cha 16 (initial)
Str 12, Dex 14 (18), Con 18 (22), Int 16 (22), Wis 8, Cha 26 (32)
Base Saves: Fortitude +8, Reflex +12, Will +12
Saves: Fortitude +19, Reflex +21, Will +16/28 (+1 vs. spells/Sp)
Class: Social Intuition, Trapfinding, Inspiration (3 points?), Cunning Insight, Cunning Knowledge, Arcane Dilettante, Brains Over Brawn, Motivate Dexterity, Evasion, Uncanny Dodge, Spell Sense +1, Doppleganger Insights (+2 Bluff/Intimidate/Sense Motive), Thought Tricks, Anticipate (+5 AC/attack vs. foes he is mindreading), Detect Thoughts at will (DC 30 +4 (monstrous) humanoids), Change Shape, Combat Telepathy, Spherical Detect Thoughts, Multiple Surface Thoughts (4 creatures), Instant Mindscan,
Feats:
1 Nymph’s Kiss
1 Intimidating Strike [Rogue]
1 Darkstalker [flaw]
1 Sacred Vow [flaw] (or Able Learner?)
3 Force of Personality
3 Skill Focus (Diplomacy) [Marshal]
6 Persona Immersion
6 Combat Panache [Rogue]
9 Imperious Command
12 Ability Focus (Detect Thoughts)
13 Quick Change [CT]
15 Fade Into Violence
15 Racial Emulation [CT]
18 Vow of Nonviolence (+4 save DC vs. humanoids, monstrous humanoids)
20 Daunting Presence (DC 33 +4 (monstrous) humanoids) [Rogue]
Thought Tricks (DC 28 +4 (monstrous) humanoids; HD 20 or less):
Daze for 1 round (Swift; Will negates)
Disrupt casting or similar action (Immediate; Concentration negates)
Confuse for 1 round (Swift; Will negates)
Stun for 1 round (Swift; Will negates)
Skills (222 points):
Balance +25 (5 ranks +4 dex +6 int +8 cha +2 synergy)
Bluff +40 (23 ranks +8 cha +3 circlet +2 kiss +4 racial)
Concentration +21 (10 ranks +6 con +5 competence)
Diplomacy +45 (23 ranks +8 cha +4 synergy +3 circlet +2 kiss +5 feats)
Disguise +48 (23 ranks +8 cha +2 synergy +3 circlet +2 kiss +10 change shape)
Hide +28 (10 ranks +4 dex +6 int +8 cha)
Intimidate +42 (23 ranks +8 cha +2 synergy +3 circlet +2 kiss +4 racial)
Jump +25 (5 ranks +4 dex +6 int +8 cha +2 synergy)
Move Silently +28 (10 ranks +4 dex +6 int +8 cha)
Perform (Dance) +18 (5 ranks ++8 cha +3 circlet +2 kiss)
Sense Motive +11 (8 ranks -1 wis +4 racial)
Sleight of Hand +21 (1 rank +4 dex +6 int +8 cha +2 synergy)
Tumble +33 (8 ranks +4 dex +6 int +8 cha +2 synergy +5 competence)
Use Magic Device +36 (23 ranks +8 cha +3 circlet +2 kiss)
Skill Tricks: Quick Recovery, Never Outnumbered
Languages: Common +6
Equipment:
Veil of Allure (face; +2 save DC for all cha-based abilities)
Circlet of Persuasion and Int +6
Vest of Resistance +5
Minor Cloak of Displacement and Cha +6
Con +4
Dex +4
Slippers of Battle Dancing and Agile Leaping (Stand as swift w/o AoO; Dex on Jump checks; +10 ft land speed; +5 tumble; +2 init; use Cha instead of str/dex on attack/damage w/ 1H or light weapons if moved 10+ ft) 34650 gp
Ring of Freedom of Movement and Mind Shielding
Novice DM Ring of Protection
Tunic of Steady Spellcasting
Tome of Leadership and Understanding +5
