Some of the US border guards at Point Roberts are pretty funny sometimes. But the idea is to joke only if they initiate.

One time I was picking up a special room circulation fan for the GF, told him it was for my gf and he was like "why, does it often get hot at her place?"

and then recently picking up some dresses she bought online, the guy was like "these are for her hopefully, right?" jokingly, and I'm like "well... for the most part"

Women border guards act more bitchy in order to compensate for being a chick I find.

I've only ever had one Nexus border agent be funny. He asked me who I had in the trunk (in a joking manner). As much as I wanted to joke about it, I didn't...he also made a comment about whether I had any Tim Hortons coffee. Other than that, they usually just ask where I'm going or if I have anything to declare, or sometimes nothing at all.

they just ask you questions to see how well you answer them, not because they're interested in the answer.
they might ask, where do you work? how do you know each other? what kind of car parts did you buy? what brand were the shoes you bought?

they just like to hear how calmly and confidently you answer their questions.