The thing everyone forgets? Once you've left the car deck, you might not be able to nip back for a charger or a jumper.
The little bag is the important one
Think of the items you'd be annoyed to lose access to for the whole crossing: tickets, documents, phone, charger, medicine, water and something warm. Put those together before you queue to board.
Driving doesn't mean taking nothing onboard
The car is brilliant for storing the big cases, but it may be inaccessible at sea. If you're travelling with children, add snacks and spare clothes. With a dog, take whatever the ship's pet arrangements allow and require.
For a night at sea
An overnight bag with toiletries, fresh clothes and your sleep essentials makes a cabin much easier to use. Check the operator's luggage rules if you're travelling on foot or bringing anything unusual.
Routes to compare
A few things you might be wondering
Can I leave luggage in my car?
Leave non-essential items there, but do not rely on returning to the deck.
Can I bring food?
Rules vary by operator and route; check the current guidance.