The Jolly Balloon Man in exile (exiled by Agatha's Soul Cauldron).
All other permanents on the battlefield.
Midnight Guard has at least one +1/+1 counter on it.
Midnight Guard does not have summoning sickness.
({1} magic symbol) available.
Steps
Activate Midnight Guard using The Jolly Balloon Man's ability by paying ({1} magic symbol) and tapping it, creating a token copy of Priest of Urabrask that is a 1/1 Balloon with flying and haste in addition to its other types.
When the Priest of Urabrask token enters the battlefield, it and Midnight Guard trigger.
Resolve the Priest of Urabrask token's trigger, adding ({R} magic symbol) ({R} magic symbol) ({R} magic symbol) .
Resolve the Midnight Guard trigger, untapping it.
Repeat.
Notes
Once you have infinite mana, you may target any other creature using step 1 instead.
Results
Infinite LTB.
Infinite ETB.
Infinite sacrifice triggers.
Infinite death triggers.
Infinite creature tokens with haste.
Infinite token copies of each creature you control with haste except Midnight Guard.