There at least two points when I thought of an answer that seemed better than the ones Obama gave.
Obama seemed to keep a theme on Romney having a history of being vague (though not a very strong one as Obama maintained no strong themes). When education came up and Romney said he would not cut spending on education I would have pounced, because up to that point he had already denied planning to cut so many things. If I were Obama, I would said that if tax cuts and spending cuts were so important to Romney's plan then he should actually tell us what he is going to cut instead of everything he somehow isn't.
The other thing is, as sort of highlighted already, I would have attacked his economic plan. I would have pointed out that without the "accounting for growth part", it wouldn't add up at all, and the "accounting for growth" part is basically an insubstantial, magical non-answer somehow expected to hold his whole plan together. It's like the underpants gnomes.