Air filter & Radiator coolant
Air filter and Radiator coolant.

Screen Washer; Check every day for washer level.
Expansion Pipe; This is the overflow pipe, and expansion outlet for the Radiator.
Radiator coolant. Check this is not blocked and check the outlet, situated on near side chassis. If you have radiator problems, it is here you will see stain marks of excessive overflow.
The days of cheap antifreeze top-ups are long gone. High Spec antifreeze transfer heat more effectively and at the same time protect against corrosion particularly aluminium.  If while checking out your coolant, or is older than 2 years.  Then your anti-freeze recommended properties will not be good enough. The properties of Radiator coolant also include the ability to stop rusting internally and has lubricating qualities.  These properties are not in the anti-freeze after 2 years. It is not advisable to mix modern antifreeze coolants.
To ensure no air locks when refilling with new anti-freeze, I normally let the van dealer do this work. The proceedure these days is to suck out and replace, elliminating air locks.
From cold, if your coolant is at the MIN. level don't top up as the level will reach the MAX. when the engine is hot.
Air filter; Not something you would check often, but it is useful to know where it is and to check it occasionally and before you have the vehicle serviced. [Will it be changed?]
Headlamp Adjust; This pipe is factory filled with hydraulic expansion fluid. check for leaks and whether the headlamp can be adjusted when you alter the dash switch. Due to it's position behind the headlamp, it is easily caught and ripped off. [Perhaps a modification to cover these valves at the back of the headlamps could save future expense.]
Coolant Return Pipes; Important, Always check the coolant return pipes for cracks and leaks, and vibration scoring.
Engine Air Input Pipe; Check for cracks and splits and ensure the the pipe "Jubilee" clip is secure at both ends.
Headlamp; Check your lights every time you take the vehicle out. Check the headlamp for dipping. You don't want to be one one of those people running around with only one headlamp working.

Tank Level Monitor

This type can be found in most vehicles. It comprises a float on the body. This float is magnetic. When the float drops, due a water fall, the magnet pulls in a miniature Reed Relay, located inside the body, which will turn on a monitor in the dash board. This device is very reliable and has been on my Boxer from new.

In the Boxer this is a lamp underneath the Speedometer.

Expansion Tank level Monitor, found on the Expansion tank.

The magnetic float can be seen in White and is in the 'No water position'

Level Monitor showing the reed switch inside.