Here’s a few things to consider.
Work out what the various bits do that come in plastic bags, and think where to store them. E.g. table support strap, emergency hold down kit for roof, 12v socket torch, anti tamper wheel nut.
The dealer provided us with a little bright yellow pouch for our wheel nut which we keep in the glove compartment along with the water cap key (a tip from friends who rent out their van, so no one ever gets confused between the water fill point and diesel fill!).
Find out where the wheel jack is, and depending on your wheel size, the compressor and foam gadget or your spare. If you’ve a spare, consider fitting a lock.
“Lubricate” the sliding roof hatch door. Ours was very stiff from the off. It should move easily as otherwise you’ll be cursing it to open, and if it flies open suddenly you risk breaking the end of the track where a screw and nut acts as the end-stop (like we did - since managed to fix it). We use Autoglym plastic and rubber cleaner - works a dream and it isn’t messy.
Check the dealer has filled the ad blue tank. Ours didn’t so we had about 2k range. I filled it and we then had 18k range - enough for a year or more without worrying.
If your window blinds are stiff to pull down - DO NOT try and lubricate the mechanism in any way - you’ll regret it (yes, been there done that!).
Get some protection on the VERY expensive folding topper that you’ll no doubt have got for the downstairs bed and sits on the multiflex board. A replacement would set you back lots of dosh so it pays to protect.
In the boot - find some foam sheeting etc to protect the sides of the van (easily scratched surfaces) from whatever you put in the boot, and some mats for the floor to stop stuff shifting around.
In the cabin, get a rubber mat set. Not glamorous but very practical / necessary in our view.
Get rubber inserts for the front doors steps and the sliding door step. The light grey plastic gets easily scratched and scuffed without them (a van we had hired looked pretty beaten up and it had only done 1600 miles). Double sided tape to fix.
Seat protector covers? Your choice.
Fridge bottom ‘trivet’ - cheap via amazon - see forum posts for link.
Flush out fresh water tank well. This also checks the tank drain has been properly aligned in the factory and it doesn’t flood your boot.
There’s probably more , but the above comes from our experience.