I've liked having the whole relationship between the Doctor and the Ponds explored - the effect he has on their day to day life and their feelings about it.
And then of course there's the entirely freaky idea that your best friend that you grow up with could actually be your daughter. What does finding that out do to your head? If you remember kissogram Amy in the first Matt Smith episode, you can see where River got her sass from. Or did her morals and attitude from having to make her own life rub off on Amy? Nature or nurture? You could tie yourselves in all sorts of knots thinking about that.