In the bar, DiNozzo asks for sarsaparilla. This is a call back to the episode Minimum Security (2003), where this was the drink Gibbs permitted Tony to have while on duty.
Gibbs Rule 9: Never go anywhere without a knife.
During the episode, Gibbs tells Tony to "watch his six", its military slang meaning "watch your back", using the numbers on a clock face to indicate direction, like points on a compass. Twelve is in front of you, three is to your right, six is behind you, and nine is to your left.
(Probably used because all other options were taken. Compass directions are for travel, port and starboard are for ships, left and right are for vehicles on ships, so people get the clock face.)
Tony asked Kate if she still believed she looked 25. Sasha Alexander was 30 when the episode aired.
Michael Weatherly (Tony) and Jenya Lano (Carol Powers) both appeared on episodes of Charmed.