dragons mirror in game rules
Movement: Movement modification for the square grid. convert distance to number of five foot squares. diagonal does not use up two squares. that’s just annoying and something they got right in forth ed.
Grapple: In order to perform a grapple check simply make apposed grapple or str checks which ever is higher. same for continued grapple checks. defender wins on ties
Experience: Alternative Experience rules in place exampled by the one in unearthed arcana book. Modify as appropriate but the rules allow players to gain experience not just from killing things. but from solving puzzles. disarming traps. and finding alternative resolutions to the encounters. usually a base xp is awarded based on the CR (challenge rating) that the encounter is determined to be rated.
Alternate death and dieing rules: Mostly situational. but the alternate rules state that there is no unconscious at 0 or bleed out. rather you keep going until -10. this is usually dropped for original rules in cases of someone playing a bar bar or other class using the die hard or similar feet as it kinda defeats the purpose of such things.