I have an idea. 

A couple of my friends have never played D&D before, and I want to expose them to some of the more classic D&D monsters. I run a very combat heavy game, and would like them to experience what it's like to fight a beholder, a baslisk, a displacer beast, etc. Ya' know, some of the classics.

The problem is I don't have enough game time to draw them out of town and set up an encounter. So, I figured I would design a sort of arena where the players sign up to face whatever comes out of the portal. I'll have a table to roll off of, so even I won't know what's about to come out.

I know I want to do a kobold encounter, because kobolds make for hilarious combat when the two handed weapon wielder who does tons of damage still has to hit them one at a time. I want to do a golem (random which type), a giant lizard man, and on and on. 

Any ideas to add? Anything I should put in the arena so it's not just an open space? I was also thinking that the arena itself would shift inside to form a lair for whatever creature gets summoned, that way they can use their lair abilities.

easy enough. Gives you shades of gladiator stuff (recently did a lot of research into that for my own version of gladiator games, tosses in the possibility for pokemon stuff if that is a common thread in everyone's lives, and adds some excite ment through the random aspect.

set the dimension of the arena out, put faceless people that cheer in the stands, and go for it.

I like the faceless cheerers. Nice touch.

Well if we're going for more or less classic monsters, perhaps a purple worm emerges from the portal and creates a bunch of tunnels that can be used by monsters or players in the next fight.