Since you are playing on cerilia, which is more medieval than other AD&D
worlds, you should consider that the parties of elves are not welcomed much
in human lands. People in the middle ages were quite intolerant after all.

Otherwise, if your players don't mature, have them bungle a few missions just
because they can't look like humans enough to sneak around difficult

Lastly, have them meet powerful human characters specially built to to give
them quite a fight. If they really are power gamers, they should be

Horten the Mage