Do the characters have anything in common at all? Are they all boy crazy? Robots? Catgirls? Video game characters?
Have a dance off, copying a dance from a well known show, like Grease, or that Buffy thing (I don't watch, so I have no real idea about that one), or Thriller, or something... ?
Can you coax someone up to be a host on some sort of show? (We did that once. It was funny, IMHO, and the audience laughed. Our charas had NOTHING to do with one another, except that their series' were all over, which became our title: Today, on Johnny! Anime Characters Whose Series' Have Ended - Where are they Now? (AN02)).
Other people have had ideas such as "This is your brain.... at a convention, after two days without sleep, after eating 12 boxes of pocky, etc..." (AN03)
We also did a dating game style show, showing off the 'bachelorettes'. (TT03)
Um... gah. Sorry, I am not the creative one. Our best ideas came out when we were sitting as a group and chatting.
One we had but didn't use was that the anime characters were really 'live', and we were going to show them in the coffee room on a break, complaining about certain well-known things in their series (i.e. Sakura might complain that she always has to change clothes, and has no room in her closet, or that fittings with the seamstress sucks, or a video game chara might complain that what gamer would ever hit that combination of moves anyway, because then they're a button-masher, or complain about how they have nothing to do because the other color version 'player 2' is the more popular choice, so they get all the work... complain about having to wear wigs, scant amounts of clothing to appeal to the target market of young males, etc).
I think I will just stop rambling now....