So, for fun, what do you think he got her, generally speaking?
We know that it is going to be subject to some kind of delivery mixup, so I think it's probably a bit large ... my guess is that it is artwork of some kind. (.. or a dog, LOL!)
As to what she got him -- that's harder, but I think she's going to be going for meaningful rather than especially romantic, or silly fun. It won't be expensive, but I suspect that whatever it is is probably going to be symbolic of trust. Beckett puts a lot of stock in symbolic objects.
PS: I still want a glimpse of the life-sized Boba Fett in the master bathroom!
PPS: When watching the sneak peek where they all visit the crime scene, play close attention to the fliers posted up inside the bus shelter. There are inside jokes there.