I Know a Guy a.k.a The Calrissian Rule (starting at level 2)
In any situation (where it makes sense), a PC can declare “I know a guy,” and then quickly describe that NPC & the relationship. Fewer details are usually better - that leaves room for the DM & PCs to play. Upon declaration, the NPC exists. Then, when the PCs try to interact with the NPC, the player who created that NPC makes a Charisma roll to see how the NPC reacts. Depending upon the situation, the DM may decide advantage/disadvantage applies. Once used, this can't be used again until the DM determines enough time has passed.
Co-Initiative
When two or more players start combat with the same initiative, they can choose to weave their turns together. Movement, actions, and bonus actions can be taken between the players in any order. Multi-attack actions cannot be broken up.
If the players prefer to take their turns sequentially, initiative order will be determined by Dexterity
Gaining Hit Points
When leveling up, you can either roll on your hit die and take the result or use the average roll (rounded up). If you roll a 1, you can re-roll. Alternately, you can forgo rolling entirely and take the average hit die roll as listed in the Hit Points feature in your class description
Crunchy Crits
When you get a critical hit, you will roll your regular damage die, add the damage modifier, and then add the highest roll possible of your damage die. For example, getting a critical hit when your damage would normally be 1d8+7 would result in a critical damage roll of 1d8+7+8.