Recently I managed to get my party into a bit of a fun dungeon. Things are a little wacky and it's a nice change of pace from the serious stuff that has been going on before this. One of the features I've sprung on the group is a need for tickets to pass a door. The group had to backtrack, and have stumbled onto a "trap" that they quickly identified as having the spell deep slumber and another illusion spell. They made a leap of faith that they need to sit in the chairs in this room (the trap) in order to progress. This was correct, and currently they are in a (illusionary) arena where they are currently on a pseudo gameshow fighting enemies for more tickets.

Currently, I was just flipping though the bestiary coming up with appropriately more challenging encounters, but was wondering if anyone had any fun monsters that they knew of that I might bring in? We left off around the groups level (which is 8) so I'm hoping to increase the encounter difficulty form here. Killing the group is not an issue, as none of it is real and they are all going to be heal back up to being fully rested when it is over. Game is happening in 3 hours, so I will likely delete this post after that, any ideas are welcome.

tldr: Party is in illusioned arena. What are some fun monsters I could throw at them? (higher than CR8). Death is not an issue.