Some poor sod with a noose around their neck is dropped at increasingly higher heights. Bets bin in centimeters until death.
Mo-RON pit fights
Brains in jars fight in exosuits of various deadliness. The loser is killed. If the time limit runs out they are both killed. The Daddy Longlegs exosuit is the most popular form.
Dice with Death
Simple game. The Grim Reaper rolls 1d20, and so do you. Higher roll wins. Death wins on ties. Double your bet if you win. Permanent death if you lose. Lost bets go to the House.
All participants are given black clubs. The objective is to sneak up and clobber each other on the back of the head and scream "BLACKJACK!" Every person hit this way is out. The pool is split between the last 3 contestants.
An imprisoned oracle is forced to portent unimportant futures (i.e. who will be the next person to enter this room?; what color robes will the Archmage wear tomorrow?) Bets are made on the outcomes.
Interns are assigned parts in a saucy play. If they mess up the lines they usually face a shower of thrown bottles. Bets taken on how many screw-ups occur, and whether certain lines or passages can be remembered.
Contestants must read one full passage from the Book of Aman-Thurr, which bestows raving madness upon the reader. If you can prove you’re not mad, you win your bet back. If not, you have other things to worry about than your bet.
Valdo, Master of Disguise Both Magical and Not, hides somewhere in the building. First contestant to find him cashes in. Valdo will always wear some kind of red and some kind of blue.
Two contestants take turns drinking from the Grail of Trivia, which fills the drinker’s head with useless knowledge, replacing useful knowledge if their brain is at capacity. After every sip the drinker says a bit of unknown trivia (to prove they drank it), and then tries to say their name. They lose if they can’t.
Simple game: you got a mystery box with a mystery demon inside. Until observed it is both dead and alive inside the box. Open the box, is it dead? What kind of demon is it? Bets taken on demon type and whether it’s alive or dead. Typically played inside Magic Circles.
Victims are introduced to a brain parasite which causes them to start blurting out secrets one by one until they’re all out. Bets taken on number and types of secrets divulged.
Two or more contestants are given the exact same pool of potions to take turns drinking from. Vessels range from elaborate glasses to human skulls to ancient vases. Most are harmless. Some produce odd effects. At least one of them is deadly poison.
All contestants throw their bets in a pot. Last one alive collects.
A peculiar redundancy: bets are taken on the stock market: whether certain stocks will trade higher or lower in a particular period.
Nightmare Death Racing
Of course wizards bet on summoned Nightmare racing! Time spells and teleportation disallowed, but offing the competition is fair game.
Weird enough, you can buy actual for-real insurance at The Manticore, for everything from “your underlings suddenly disappearing into clouds of vapor and ash” to “the sun doesn’t rise tomorrow”. Legit insurance got run out of business long ago in Wizard City by Oracles, so this is the only place you can buy it.
Each player is given a magical bingo card chock full of Manticore shenanigans. Each entry contains such gems as: “Cause The Master of Games to lose his temper”, “Summon a demon beyond your control”, and “Vaporize!”.
Illegal Spellgold generator turned slot machine, except you play with prepared spells (or spell slots for 5e). Payouts are in Spellgold. Odds are not in your favor, as Spellgold is worth more than prepared spells.
Pretty normal poker game, except the stakes are body parts. One typically starts at extremities (i.e. ear, finger, toe) and work their way up to the big stuff (liver, kidney, eye). A wizard surgeon is generally present to ensure that contestants shed their parts safely. Winners may add shed body parts to their own bodies.
The Unsafe Bet
This is basically just money laundering, since The Manticore doesn’t deal in stolen asset retrieval (they won’t return stolen money spent on bets). Betting wizard asks the cashier for “The Unsafe Bet”, which is a near-guaranteed loss of 1-10% of what you put up. Something along the lines of “I won’t retrieve this money”, or “the sun will rise tomorrow”.