I cobbled together all of the variants and optional rules in the Player's Handbook and Dungeon Master's Guide, and put them into a single list (okay okay, I didn't do it...but the kind folks over at
this website did). Here's what my "dashboard" looks like.
Ability Score Generation (PHB 13): 4d6 Method
Alignment (PHB 122): On
Action Options (DMG 271): Off
Alternate Epic Boons (DMG 230): On
Cleaving Through Creatures (DMG 272): Off
Crafting Magic Items (DMG 128): Off
Encumbrance (PHB 176): On
Equipment Sizes (PHB 145): On
Fear and Horror (DMG 266): Off
Feats (PHB 165): On*
Firearms and Explosives (DMG 267): On
Healer's Kit Dependency (DMG 266): On
Healing Surges (DMG 266): Off
Hero Points (DMG 267): Off
Hitting Cover (DMG 272): Off
Honor (DMG 264): Off
Initiative Variants (DMG 270): Side Initiative
Injuries (DMG 272): Off
Inspiration (PHB 125, DMG 240): On
Loyalty (DMG 90): Off
Massive Damage Variant, aka System Shock (DMG 273): Off
Milestone XP: On
Mixing Potions (DMG 140): On
Morale (DMG 273): Off
More Difficult Magic Item Identification (DMG 136): On
Multiclassing (PHB 163): On*
Piety (DMG 23): Off
Planar Effects (DMG 50): On
Playing on a Grid/Using Minis (DMG 250): On
Players Award Inspiration (DMG 241): Off
Plot Points (DMG 269): Off
Proficiency Check Variants (DMG 263): Off
Proficiency Dice (DMG 263): Off
Renown (DMG 129): Off
Rest Variants (DMG 267): Gritty Realism
Sanity (DMG 264): Off
Scroll Mishaps (DMG 140): On
Skills with Different Abilities (PHB 175): On
Slow Natural Healing (DMG 267): Off; not needed with the Rest Variant I use
Spell Points (DMG 288): Off
Training to Gain Levels (DMG 131): Off
Variant Backgrounds (PHB 130): On
Variant Human Traits (PHB 31): On
Wands that Don't Recharge (DMG 141): On
-----
*at low levels only. This switch turns Off at 5th level...but honestly, I've never had to enforce them. My players tend to grab a feat for their first ASI and then buff their stats for the rest of their careers. And only one player has ever wanted to multiclass.