Theme ideas:
Sunset
King Arthur's round table
Grave Mound (the inside)
http://en.wikipedia.org/wiki/Megalith
They were normally covered with earth of course.
Ogre Sacrifice (the player gets sacrificed? or the ogres)
Grimm's fairytales (could be VERY scary)
http://www.familymanagement.com/literacy/grimms/grimms-toc.html
Quake University (teach complete newbies Quake)
Malice has something like this, there are test areas with terminals which tell you how to use the hoverboard etc (using console messages IIRC)
could be pretty funny. There's no limit to what you explain. For example you could have a giant penis and explain that to the newbie. You could also just explain weapon usage.
The Fraggles (anyone remember the series? it took place in an underground tunnel system which was accessed from a house, and had some whacky characters. I loved it.)
FPS parody (ridicule shooter games like Counter Strike, Far Cry)
Microsoft Tech Support (monsters need Windows tech support, eventually you run amok)
Fast Food restaurant (you work at the counter, your customers are monsters)
Bank raid gone wrong (either you are the robber, or a gang of monsters (they escape, you must hunt them)
Pulp Fiction (the map must start with the message "Everybody be cool this is a robbery..." and contain random violence and bloody accidents)
Spaceship service technician (cleans the spaceship of parasites while smoking weed, Fifth Element)
Smoke-Free Inc. (Quit smoking in 5 minutes - GUARANTEED) (Steven King)