I couldn't see AnotherEmma's point at first, but I can see on re-reading how you have a bit of a tone of "but what do you little people actually do with your time, surely it's terribly boring?" Rather than "it must be nice to have time together".
For what it's worth, although for a while we were desperate to be parents, it didn't work, and now instead of just seeing the loss and emptiness, I also see the years of exhaustion, boredom, drudgery and expense we've escaped too.
For us, we travel, play video games, watch movies and tv, go to theme parks, relax, go for walks, go to the gym. Sleep. Have sex. Enjoy each other's company. Laugh a lot. Whatever we feel like really.
So, imagine what you'd like to do together and then ... it would be that.