Well, if you go to the
Monsters by CR section of the PRD, you can find stuff that will be around the encounter "toughness" you want.
Just looking at CR 7 stuff and below (which is a 50/50 win situation against a level 7 PC):
A dire bear (or cave bear) has fairly even/low damage for it's level, and you can even have it muzzled or chained up so it can't grapple to make it not quite as dangerous.
Or you can take a normal bear, and give it a couple extra hit dice to make it have more hitpoints, etc.
Bear "wrestling" actually has some real-world background to it.
In that same vein (grappler), a Girallon would be another gladiator-type encounter. High damage though, especially with the potential for rend, so maybe a drugged version that weakens their strength score a bit would be appropriate.
It has the "was once possibly a noble race, turned savage" background too, which is great for a "slave fight" style of encounter.
For the "giant humanoid" fight, you could change things up and use an Ettin. Just take away his weapons, and he's not nearly as dangerous, although I'd still give him Improved Unarmed Strike to make him a threat.
Give it class levels if you want it to be tougher, but keep it unarmed so that it deals low damage.
A Dire Lion is normally a nasty encounter... but mostly because it can get a ton of attacks on a surprise round. If there's no chance to surprise or charge (chained), it's a fairly mediocre combatant. The grab might have the chance of making a bad day for a weaker character, so once again a muzzle or just a younger version that's weaker might be a good idea (the "young" template will reduce damage dice, although it does lower hitpoints and ac in the process).
Borrowing an idea from Eberron's "Six Stones", a unique odd-ball encounter could be being put into a smaller walled (but open air to be viewable) area with a cockatrice. No weapons, no armor, and see if you can survive the Dex damage and not be turned to stone!
Finally, as an "epic" style King of the Arena type of creatures... an Invisible Stalker! Yeah, yeah... kinda tough. Give it the "young" template to reduce damage, and have some kind of magical "slave brace" on it so that it can't fly (it's how they keep it from running away), and you've got yourself a terrifying fight when solo. Hell, could be doing very, very little damage and it'd still be dangerous... the PCs will be doing Perception checks to blindly swing at the creature.
Maybe make it fight in sand or water or something (difficult terrain is always fun in gladiator style encounters) or something to give them a good chance at knowing the square it's in, at least.
