Drilling your girlfriend, but not blowing your load til' after she's came. Then, right before you're "done", immediately pull out and give her a big blast to the face screaming "CASTING!" and doing the Gnome dance. Afterwards tell her you have a 30 minute cool down.
I casted Arcane Missile on Tiffany last night. Can't say she wasn't satisfied, but I think she was surprised by Insta-Cast. Next time I'll have to go Devotion Aura and cast Seal of Righteousness on her ass.