Warm Outerwear - Paddling Parkas, Fleece Ponchos, and warm hat
Put something on right after you get out of the water
SurfFur Water Parka not inexpensive but very practical in colder climates. If you have a group for a volume order they have given group discounts in the past.
If you aren't changing into dry clothes, a long parka or poncho can keep your car seat from getting wet
Warm drink - soup, cocoa, etc. in a thermos will help warm you up
Additional snack - energy bar, etc. for quick energy boost to help body heat generation
Car idling used as a luxury isn't a great practice given climate change concern, but if you are really cold - crank up the car heater and get warm. And go out and buy that electric car.
Getting Out Of Your Paddling Gear - if you are going to change clothes before going home
Bring a change of dry clothes in the car
Foam Pad to Stand on - warmer, more comfortable to stand on, protect the gear you are getting out of - keep your wetsuits clean, minimize wear from contact with rough surfaces
No Changing Room? you can use a towel or that warm robe or poncho you already put on for privacy.
Some robes like Surffur are built to assist changing clothes - roomy enough for changing clothing and provide holes in outer pockets to allow inside access.