Ha, I made that one way too vague, in retrospect. What I was thinking was, maybe, as an Auror, Tonks has access to some interesting stuff - files? personal belongings that were confiscated from Sirius en route to Azkaban? - and it might be fun if she somehow, in the course of doing her work, stumbles across some interesting dirt about Sirius or Remus or both. Maybe something funny?

But re-reading it, you could totally interpret that as something naughty, which I didn't think it necessarily had to be. Just something to get her talking to the two of them, and to get them talking about their colorful past.