The unique ideas shown here are things we don't see very often! Rare solutions, unusual improvements, and clever ideas that make nomadic living all the more fun, convenient, or otherwise nice.
Storage

If you have a section of blank wall near your door that's not doing anything (such as a door panel), take a length of bungee cord and screw both ends of it to the wall so you have a horizontal cord that pulls tight against the wall. Then, you can insert a pair of shoes with the soles against the wall, or other things like wet towels. You can also do this on the underside of your upper cabinets to store towels, paper towel rolls (keep one end of the bungee removable), etc.
Kitchens
Cabinets

Create hidden storage under your countertops.
Consider the "foam bones" technique when building cabinets and other structures in your vehicle, which uses reinforced XPS foam for structure and shape. It is lightweight and can be very sturdy, and may be cheaper than wood or other materials.
Tables

If you are adding a Lagun table mount, consider attaching the mount to the underside of the table as usual, but off center. This effectively gives you an extra length of swivel arm, which lets you to move the table around to make it closer or farther in any direction.
Stoves

(After careful consideration of the fire risk,) attach a camp stove to your vehicle door for a convenient way to use and store it.
Sinks

Connect a collapsible bowl and a portable water jug (with spigot) to create a gravity-fed faucet and sink. When not in use, the sink folds up against the water jug.
Stealth
For a stealthy van build, Built to GO! podcaster Jeff Wagg created a vent behind his NV200's rear license plate to allow for sufficient ventilation without an obvious roof vent. He covers the vent (from the inside) to prevent exhaust entry while driving.
Ventilation

To make inexpensive window screens for ventilation, you can purchase household vinyl rain gutter guards from a hardware store and cut them to fit the top edge of your windows when they are partially lowered, creating gutter guard window vents. The side of the guard that would clip over the gutter can be clipped over the edge of any window.
Solar power

To boost your solar harvest significantly, add hooks to the side of your van near the roof line or as high as you can comfortably reach. Then, while parked (only), hang portable solar panels from the hooks and connect them to your electrical system. See Side-mounted solar panels.
Large vehicles

Large vehicles may give you enough space to add wood stoves, dishwashers, or washing machines to your build.
Hitch racks

Get extra surface area at your campsite with an enhanced DIY storage box on your hitch rack. Use gas struts or another mechanism to allow raising the surface of the table while allowing access to the inside.
Nomad art

Enhance your mobile living quarters with some nomad art on the inside or outside of your vehicle.
Tents

If you sleep in a tent in areas where bugs are a nuisance, consider setting up a screen tent over your tent. This keeps bugs out of your tent when you enter and exit, reduces rain noise, and gives you more bug-free space right near your tent.
Plants

Create a rack for your plants that is mounted directly in front of your windows. This keeps them safe during transit and pots can be removed when needed.