Captain Jack should guest
In the first season of Doctor Who, Captain Jack's debut was quite an introduction and shows that with a minor tweak to a SJ script his more overt flirting can be toned a bit. Its this Captain Jack portrayal that could work well on SJ.
Captain Jack could end up in SJ's neighbourhood chasing down an alien and, inevitably, the SJ gang help out. John Barrowman is a great character actor that can play different layers of Captain Jack. It would be quite funny to see SJ, Rani & Rani's mum to be totally charmed in his presence with Clyde, Luke & Rani's dad wondering what the fuss is about.
Now I know Captain Jack left Earth at the end of "Torchwood" (season 3) but these characters are quite resourceful when returning back. I enjoyed the return of the Doctor in "The Wedding of Sarah Jane" and realise that Captain Jack would make an excellent guest appearance. He would be quite aloof at first given recent events in Cardiff but, given this is a PG show, would warm to the kids eventually (Luke did help out in the season four finale of Doctor Who)