What are these elements on the back of a Boeing 737?
① Main outflow valve door


(Source)

A valve used for controlling the pressure in the cabin. Air is pushed into the cabin by the air conditioning packs, and exits by this hole. The flow can be adjusted using a valve which is located behind this door.


(Source)

The valve opening is controlled by the pressure controller when the automatic mode is selected, and by the crew when the manual mode is selected.

The design of the door includes teeth to decrease the aerodynamic noise, by better mixing air from the cabin and air ram air.

For more details about the air system, see this answer.



② Safety pressure relief valve


(Source)

This is one of the two safety valve used in case the main valve mechanism is jammed or ineffective. It's like a safety pressure relief in a pressure-cooker.

It limits the pressure within the cabin below a maximum value, to prevent pressure to damage the cabin vessel and/or create discomfort for the passengers.



③ Drains


(Source: Question)

Unknown function drains. I believe they are used to evacuate flammable fluids leaking from the APU.



④ Tail-strike assembly:


(Source)

This tail-skid is used to limit the contact between the aft section of the fuselage and the runway. It absorbs some of the tail strike force. While I'm not sure for the B737, on other models (B757, B777) the tail-strike assembly contains a sensor to alert the crew of the strike occurrence, by displaying a message on the EICAS (thanks to @RalphJ for questioning).


(Source)

The intensity of tail strike force is registered by the color indicator. A force higher than the maximum permissible hides the green strip and requires a maintenance check before the next take-off.

This strip is checked during the pre-flight inspection (walk-around) to prevent flying with a possibly damaged aircraft.



⑤ APU drain mast.


(Source: Question)

Drains oil from the forward accessory and the compressor bearing of the APU.



⑥ Aft drain mast.


(Source: Question)

Heated drain mast, part of the water system, and used to evacuate water and waste from the aft galley and the lavatory wash basins ("gray water"). Schematics of the B737 NG water system:


(Source: B737 NG General Familiarization)