Suppose that, on your turn, you ready an action to hit the enemy in front of you if he tries to hit your friend.
What else can you do in the turn you ready the action?
(I'm asking about the turn during which you decide to ready the action "hit the enemy" - not about extra things to do as part of the readied action itself, e.g. "hit the enemy and then move away".)
Can you, for example, move close to an enemy and then ready the attack if he hits your friend?
Can you move closer to an enemy, hit him with your offhand weapon (bonus action) or use some other abilities (Second Wind for fighters) and then ready the action to attack if he hits your friend?