When I’ not running Macchiato Monsters or a new game I want to try, I use what Jeff Rients calls walking-around D&D. Which means, I use the mechanics I can hold in my head, and adjudicate the rest. For future reference, here is what I tell people who join a Flailsnails or generic D&D game I’m hosting. I’ll update this post as the rules see more use (edits at end of post).

 Your character
– Keep your stats and hit points, unless they’re too different from TSR standards and I make you roll 3d6 six times (and arrange the scores to suit your character).
– Mods are:
4-: -2
5-7: -1
8-12: 0
13-16: +1
17+: +2

– Your Save is 18 minus your level.

Applying violence
– Group initiative on a d6, rolled every turn
– You do ONE thing every turn (including move or swap weapons)
– Attack with d20 +stat +class bonus (your level, half level, or nothing). Monsters just add +HD
– Ascending AC: light armour 13, heavy armour 15, +2 for shield, +magic. Natural AC classes have 13 +stat bonus
– Damage as weapon +STR in mêlée, +magic
– Missed attacks do 1 damage. Referee may offer a ‘devil’s bargain’: attacker rolls damage in exchange for a complication (someone loses footing, a shield is broken…). Players can always refuse

Magic
– You keep your spells and magic system with the referee’s approval
– If you don’t have one: you get one slot for every level you have. Memorize one spell per slot. You can always cast a non memorized spell, but it costs time, components, or is very risky (save to avoid losing d6 stat points per spell level).
– Spell lists are either tied to spell books (and you can find more adventuring), divine powers (fixed list), or acquired disciplines (for psions and the like).

Exploration 
– Exploration turns last an hour. One character can thoroughly explore one room or corridor in that time. Divide 60 minutes by the number of active characters – a party of six members explores a room, finding everything there is to find, in 10 minutes. I draw and fill in clocks to keep track.
– Non-exploratory movement depends on encumbrance, leg size, and endurance. Rolls may be involved.
– Every hour in the dungeon, and every 8 hours in the wilderness, the referee rolls an event d6 (usually: 1 nothing, 2 resource, 3 fatigue, 4 morale, 5 clue, 6 encounter).

Task resolution
– The conversation determines what is possible and what isn’t. Time and noise are factors.
– Otherwise, you need to save. Against death or disease of course, but also against looking like an idiot or making a ruckus. Add the relevant stat modifier, and your level if using a class or race ability. A modified roll over 10 always means a partial success or hard choice.

Classes
– There is always the option to level up using Zak’s system
– For new characters, here are the basics:

Class  =>       Level	Fighters   d6+1 HP   Kills; +lvl doors/gates or intimidation	Rogues  d6 HP   +lvl to thievery or acrobatics or speed	Wizards d6 / 1 HP     Spells or psionic powers	Mystics  d6 HP   Turn undead or command spirits	Demihumans   d6 HP/level   +lvl vs Magic or +lvl to hide or some other skill
1	0	0	0	0	0
2	2,000	1,200	2,500	1,500	2,200
3	4,000	2,400	5,000	3,000	4,400
4	8,000	4,800	10,000	6,000	8,800
5	16,000	9,600	20,000	12,000	17,000
6	32,000	20,000	40,000	25,000	35,000
7	64,000	40,000	80,000	50,000	70,000
8	120,000	80,000	150,000	100,000	140,000
9	240,000	160,000	300,000	200,000	270,000
10	360,000	280,000	450,000	300,000	400,000

Experience
– Is mainly gained with gold retrieved from ‘unlawful ‘areas (1gp=1xp)
– Is always divided equally, including henchmen
– Goal xp aren’t entirely unheard of. Here is the xp table I’m using in my office lunchtime game:

Discovering a point of interest: 500
Unearthing lost knowledge: 500+
Making contact with a new tribe: 1000
Treasure: 1/GP (including information sold)
Eliminating a threat to others: 100/HD (non threatening monsters are worth 10/HD)

Edit 11/4/18: Added/reorganised the classes and experience paragraph based on my current, non-Flailsnails campaign
Edit 15/2/18: made task resolution/saves clearer in an attempt to stop using target numbers when I run. Added a list of XP values.
Edit 10/1/18: removed task resolution with chances in six, replaced by Into the Odd style save when there is danger. Removed the ‘yet untested’ mention about exploration rules.