Starfleet has always been about exploring, and looking for similarities among differences, and embracing that which is unknown. As a result, from time to time, Starfleet officers are offered a rare opportunity- the chance to serve, in a temporary capacity, aboard a ship or instillation from another power. This is a truly special opportunity- representing the best of the Federation, while working on bridging cultural divides, is a prospect suited to only the finest officers.
Naturally, your characters would all qualify (wink).
So the question I have for you is this; what alien service would your character be most interested in joining? Would the confined, barren darkness of a Klingon warship suit their fancy? How about the spartan interiors of a Jem’Hadar beetle?