"The perfect pet at the perfect price -- guaranteed." Creature Dex at the bottom of the sub, if you want to include "relatively harmless" alien critters in your campaign.
The PCs find a strange helmet, when donned it will take control of that PC and speak and act trough him/her. It explains that it is benevolent and needs their help in a quest and that it will stop possessing the PC when the task is done. Great if a player is gone for a number of sessions.