Use of a slackline as a training aid for both of them. Coaching each other, mocking, teasing, or one having more experience than the other. I imagine Clint has a little tight-rope experience? And it seems like something Natasha would just be awesome at from the get-go.

For inspiration: http://www.youtube.com/watch?v=eocvfjGAbDA,
http://youtu.be/YkGtxuxLFr4