Well one thing I did for instance was set up a Wave shared with my wife, in which I put a list of trips we wanted to do, with a suggestion when, or a date for when we last went on that trip. Being accessible in realtime to both of us anywhere and being replayable, it gives a nice overview of what trips we planned when, how often etc, and is a good resource for if we want a quick suggestion of what to do with a weekend.
I'm not saying it's a great or brilliant idea, but that's the kind of thing it can be useful for.
I definitely would like to use Waves instead of email threads as well, but for that it needs to be more integrated with email first, or else no-one will use it.