I like the idea of putting an innocent in harms way and letting the Paladin deal with that. Another interesting gambit, ...put him up against a Giant with a Tower Shield, and put a victim on the Tower Shield. Now he has to kill the giant while avoiding incidental damage to the "shield maiden." Rack up a good penalty on his attacks and let him take a few rounds trying to work his way through it.
Another alternative, I've enjoyed using on occassion, is obligating the Paladin before the fight. This can be done in conjunction with saving someone. The price of releasing a prisoner could be his word that he will stay out of the fight, or that he will not fight the main enemy, etc. Of course, it would help if you then provided the character with something interesting to do during the fight, or even worked in an angle where he could pursue a side combat. That way the player isn't totally bored. But the idea here would be to force him to stake his honor on his willingness to let the others take care of the fight. If he changes his mind, and decides to fight, then he loses his Paladin abilities anyway, and hence, he's easier to deal with.
One other angle to the oath-out variant is that the wording could be sufficiently narrow to allow the Paladin to help out in the fight, but not to fight himself. At 12th level he will have good curing abilities and some spells. The character could then try to use his Paladin as a support character, giving him a role-reversal (good drama) and potentially a crucial role after all, just not one that utilizes his fighting skills.