Unless there are unique circumstances, all of the showups one-shot games rely on these house rules.
There are a few notes with respect to character creation.
Alignment is dead. Unless it is important for your character to have one, please feel free to choose “Unaligned” as your characters' alignment.
The heroes are all awesome. Please choose from feats and feel free to use the alternate human character heritage as well as the alternate heritage rules from Tasha's Cauldron of Everything.
Between adventures, characters may progress in levels. For each GP of treasure taken from the wilderness and returned to civilization, the party gains 1 XP in a pool split evenly between the PCs, typically at the end of the session. Magic items and items that require appraisal do not count toward that total.
Every time an adventurer becomes unconscious as a result of dropping to 0 hit points, they suffer one level of exhaustion.
Rests shouldn't end the momentum of a game. I've made some changes to the rules for short and long rests to maintain that. Find the updated texts of these rules below with the changes in bold:
A Short Rest is a period of Downtime, at least ten (10) minutes long, during which a character does nothing more strenuous than eating, drinking, reading, and tending to wounds.
A character can spend one or more Hit Dice at the end of a Short Rest, up to the character’s maximum number of Hit Dice, which is equal to the character’s level. For each Hit Die spent in this way, the player rolls the die and adds the character’s Constitution modifier to it. The character regains Hit Points equal to the total. The player can decide to spend an additional Hit Die after each roll. A character regains some spent Hit Dice upon finishing a Long Rest, as explained below.
A character can’t benefit from more than one Short Rest in an 4-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.
A Long Rest is a period of extended Downtime, at least eight (8) hours long, during which a character sleeps or performs light activity: reading, talking, eating, or standing watch for no more than two (2) hours. If the rest is interrupted by a period of strenuous activity - walking, fighting, casting spells, or similar adventuring activity — the characters must begin the rest again to gain any benefit from it.
At the end of a Long Rest, a character regains half their lost Hit Points. The character also regains spent Hit Dice, up to a number of dice equal to half of the character’s total number of them (minimum of one die). For example, if a character has eight Hit Dice, he or she can regain four spent Hit Dice upon finishing a Long Rest.
A character can’t benefit from more than one Long Rest in a 24-hour period, and a character must have at least 1 hit point at the start of the rest to gain its benefits.
For many games, the player characters may find that it is nearly impossible to get a long rest away from civilization. Be warned.
This campaign world uses a twist on the flanking variant rules that follows the 3e flanking rules rather than the 5e alternate rule.
When making a melee attack, you get a +2 bonus to hit if your opponent is threatened by a character or creature friendly to you on the opponent’s opposite border or opposite corner.
When in doubt about whether two friendly characters flank an opponent in the middle, trace an imaginary line between the two friendly characters’ centers. If the line passes through opposite borders of the opponent’s space (including corners of those borders), then the opponent is flanked.
Exception: If a flanker takes up more than 1 square, it gets the flanking bonus if any square it occupies counts for flanking.
Only a creature or character that threatens the defender can help an attacker get a flanking bonus.
Creatures with a reach of 0 feet can’t flank an opponent.
Consuming a healing potion can be done as a bonus action. Other potions and administering a healing potion to others takes an action. Healing potions consumed as an active action heals the drinker for the maximum amount possible on the roll. For example, a healing potion for 2D4+2 will always heal 10 hit points if consumed as an action but be the variable rolled amount if consumed as a bonus action.
Be warned, player characters die in D&D games. Players of dead characters are welcome to help play monsters or NPCs for the remainder of the session. However, at their first opportunity, the player may choose to create a new player character starting with the same XP accumulated by the previous character. In these cases, we'll probably have a cut scene introducing the new player character to explain their arrival.
The text of the Counterspell spell should be changed to this:
You attempt to interrupt a creature in the process of casting a spell that takes an action to cast.
Make a contested check against the original spellcaster using the spell attack bonus (Spellcasting Ability modifier + Proficiency Bonus) plus the spell level of the original spell or counter spell. If the counterspeller's check is higher the creature’s check the spell fails, and they lose the spell slot for the failed spell but not their action. However, if a creature's spell is countered they may only use their action to cast a cantrip instead.
Spells that take a bonus action or reaction may not be counterspelled.