There is nothing in the Laws of the Game on this. Specific competition rules and refereeing policies may dictate restrictions on the refereeing team. Obviously, at all levels of the game, nothing should be done that results in any perceived bias - e.g. inviting the manager from only one team into the referee's dressing room, holding private discussions with one team only.
At most levels of the game, time permitting, the refereeing team will return to their dressing room, rehydrate, and rest. They will discuss any decisions and provide each other with feedback on performance. Also they may consult with any stadium officials about issues that need ongoing management such as spectators or the condition of the field. They may also receive advice on substitutions any managers wish to make at the start of the second half, and inspect the equipment of these substitutes outside the dressing rooms, to save having to do it when everyone is back out on the field. Les Arbitres, a documentary from Euro 2008 contains a number of scenes showing the typical halftime activities of top-flight referees.
As far as I am aware, at the elite levels of the game, decisions are regularly discussed, but video footage is not used. Knowing that a match-changing call was definitely wrong could have a crippling effect on performance for the rest of the match.
For novice referees, referee assessors may speak to the refereeing team at half time about minor adjustments to things such as signalling and positioning. Discussions about decisions are generally left until the end of the match to prevent putting new referees off their game.