Door pockets from other cars (eg Lada 2108)

Seat Holder-upper

'70s Ford "eyeball" heater vents - fabricate in for better flow control

Modify hinges to fold seats into a bed - here's how

Velcro straps to utilise the gear shift area as effective storage (JamesLaugesen)

Light switch swap mod (Mk1 only) - here's how

Bottom of the steering-column cover catch your big feet when you change gear?
Mine did. Not any more...

Opening rear windows - available from Brazil

Rear organisers

Rear shelf - converts to handy outdoor table

or go the whole hog if you've got a long-door model

Rear draws - if you're lucky enough to have a long-door model

Rear badge/indicator swap

Electric radiator fans - with a kill-switch for fording - here's how

Under-bonnet spade

Or external for better off-road access

Splash-resistant dizzy - or a rubber glove can be used to for better water-proofing:

Headlight water proofing - silicon sealer and garden hose

Rear wheel spacers - could even be made from a machined down hub

Big battery (RHD) - will fit in, but won't sit neatly on the standard tray and may need slightly taller threaded rod to hold it down

Cabin guard - stops your recovery gear etc squashing your head in the event of an off

Fire extinguisher - highly recommended to have mounted in an easy to reach (by driver and passenger) location

Under-bonnet compressor - for off-road inflation

Coil retainers - if your dampers are too long and coils aren't captive.

Tool kit in front spare tyre - a hat-box or gold pan is perfect

Or if you've shifted your spare tyre make use of the space with a big custom tool-box. Nuclear waste theme optional.

Themed tool-box - a few bits of wood and you have a 'box that is a heap cooler than an over-priced shop one.