This is a small list of optional rules I'm using in my current 5E D&D game. If we add more I'll update.
1. If players can take extra time and are not in eminent danger players can choose to "take 10". Taking 10 as their d20 skill roll rather than rolling.
2. Players can choose to apply either proficiency bonus or proficiency dice roll. Their choice.
3. On a critical hit in combat apply maximum damage for the weapon (8 on a d8) then roll the damage die and add it before adding bonuses.
4. When players would normally roll hit points (on a new level or healing) and/or weapon damage die they may instead choose to take the average roll instead of rolling.
5. Players may choose for their characters to fail at something rather than rolling to add a complication to the story. If they do so they gain inspection.
6. Optional rules that seem assumed but I'm not using. No multi-classing. And characters are limited to two feats, three for variant humans.