Who's casting those enchantments?!

So, Subtle Enchantments. That's fun.
http://www.d20pfsrd.com/feats/general-feats/conceal-spell
Deceitful, Bluff, Sleight of Hand, and Disguise. Deceitful is also required for Subtle Enchantments
Improved Conceal Spell if you want the concealment to land more consistently for bigger spells; unnecessary if you just spam 1st level spells on this. I imagine save DCs would need a little buffing on occasion, just to improve the likelihood of not messing this up, and you're going to want to leave for another town from time to time, since eventually somebody's gonna notice...

Ordinarily, you would cast a spell, the spell would be noticed and might even provoke an attack, and if the target saved against an enchantment effect they'd notice. With this combination, you're a fine bluffer and dastard, you can usually end a failing negotiation with a surprise fireball, and your Charm Person spells not only won't be recognized consistently until you're done with them, but might not be noticed at all.

Bet you dollars to donuts a psychic would have a ball with this.

Synergy:
Magical Lineage: Charm Person. It's not even an unlikely fit for a magical lineage to have developed techniques for it. Charm Person is one of those everyday life spells which can get someone in a lot of trouble, but which is easy to fall back on; by comparison, normal people don't spend a lot of time working out the best way to cast Flaming Sphere.
Jinxed Spell. Halflings get a charisma bonus, so this is best suited to a class that needs charisma anyway, such as psychic or sorcerer. Arcane bloodline seems a decent fit, since a halfling sorcerer with Jinxed Spell is kind of a metamagic-heavy build anyway, and you can do more with that via the 3rd level benefit from Arcane Bloodline. What I can't figure out is if they automatically notice the jinx. I rather doubt it.
Fleeting Spell. It's a +0. It's actually a downgrade for most purposes, but the spell cleans up after itself, making it harder to notice it was ever there, and you can dismiss it nigh-instantly whenever you need to. Obviously this does nothing with Magical Lineage, but that just means there's room for another metamagic feat.
Stylized Spell. A +1 enhancement that fits well with Fleeting Spell; the spell's harder to identify. The options are impressively broad, Magical Lineage drops it to a +0 modifier if you apply no other metamagic, and the requirements aren't out of the way. My favorite part is that, when the rest of your obfuscations fail and they notice you're casting a spell, they can still be wrong about *what* you're casting.
Persistent Spell. Since successfully hiding the spellcasting relies in part on successfully landing the spell, requiring them to roll twice to fail the save means you're more likely to not have to worry about all the obfuscations falling apart. It's a +2, but if you're spamming Charm Person with Magical Lineage, it's just a +1.
Quicken Spell. Conceal Spell increases casting time *unless* casting time is a swift action. This allows you to cast most spells with far shorter interactions. At +4 it's a lot, but it probably does wonders for a spontaneous caster.
Studied Spell. This is a nice metamagic feat for this kind of concealed social casting. Elves and a few other critters have on saving throws simply as a result of their race. Defeating those saving throws is very helpful, and might be worth the +2, or the +1 with Magical Lineage.

If you're a spontaneous caster and you're using metamagic, that's a full round action. Good thing you have Conceal Spell; the casting time increases to *two* full round actions, but all you have to do is keep talking for a little longer.

Oddly enough, there's another feat: http://www.d20pfsrd.com/feats/general-feats/cunning-caster/
The success rate on the bluff check is lower, but there's no indication it doesn't add a straightforward level of obfuscation, and its requirements are, again, not far out of the way. The main problem is how many feats you're now throwing at the (admittedly very important) benefit of nobody noticing you're being horrible.
Much of this can be simulated by Spellsong. There's not a lot of synergy; they have different requirements. I can definitely see an argument for Perform (Oratory) or Perform (Acting), though, in which case your gestures and speech in a Bardic Performance would allow you to Conceal the Spell, and the Swift Action to cast during the BP wouldn't be increased by Conceal Spell.
It's a lot of effort and feats, but at least the skills aren't too out of the way for a bard.

I think I'm going to build this character. Magical Lineage for one or another enchantment spell, spontaneous caster, Deceitful, Spell Focus (Enchantment), Subtle Enchantments, Fleeting Spell, Stylized Spell, Conceal Spell, and Cunning Caster. Studied Spell and Persistent Spell might occasionally be important, as might Tenebrous Spell for its simple increase in save DC if you're not too worried about delivering the spell safely in a dark alley.