I think Allen West would be a great choice, objectively speaking. He’s just as assertive as Christie, and even more accomplished.
Romney would also shore up any reluctant conservatives.
Personally, I think that is Romney's only hope of getting out the base. If he nominates a true small government VP then it would be a "for" vote rather than a "not the other guy" vote.
Rand Paul would be another option but the West - Biden debate would be most entertaining