Original Commentary
The fact that Kimberly is playing along with all this (for more than one panel) surprises me too. This story arc actually began with the Sunday comic on the previous page.

Modern Commentary
They both know what's going on here. Nobody's fooling anyone and nobody's trying to fool anyone. But you can't just admit that you actually hadn't already bought a gift.
The "it's something you don't like" is, of course, to head off any questions about why he didn't get Kimberly anything.
Are there foods that he likes but she doesn't? Almost certainly. I don't that he could name one at this point in their relationship and the generic "something you don't like" is funnier than explaining it. - Terrence