Sleeping really, really well "downstairs" in the California Beach.

VW sells an optional "comfort mattress", a 5cm cloth-covered piece of ...foam for a few hundred Euros. Even the VW sales people are reluctant to sell it. According to pretty much anyone who has bought one, it is not comfortable. This is what is looks like: https://shop.ahw-shop.de/original-vw-t6.1-california-beach-bettpolster-comfortauflage-matratze-klappbar-camper). This money is probably better saved and put towards a quality mattress.

As our junior sleeps "upstairs", and the new bed springs upstairs in the T6.1 is quite comfortable already (at least to us, I weight around 72kg, might be different if someone weighs a lot more and wants to sleep upstairs).

There are a few options out there, which actually deserve the name comfortable, and are still stow-able in a decent way, here are some favourite options:

The maker of the Calicap has a few options, both for the lower and the upper bed:

https://shop.vwcamper.de/schlafkomfort/

Other manufacturers are:

Optima: https://www.optima-schlafsysteme.de/klapp-und-falt-matratzen/volkswagen/t6.1/

Matrazen-Lab: https://matratzen-lab.de/category.php?id_category=83

We opted for another option though. Based on some research, we contacted a guy  known simply as "Werner" in the local forums. Werner makes mattresses by hand, and tailors them to the requirements of the van and its "inhabitants". You can select to have extra soft/supportive sections on one or both ends of the mattress, and it is very pliable so it can easily be folded or rolled and stored on the multiplex board when not in use without hindering the view backwards. We got the full-size 150cm wide mattress and can still easily access the window storage units (we have the ones from Qubic, the other well known brands are Vanessa and Felleisen).

To contact Werner, contact us and we'll happily forward you his address, or check the Caliboard forum in Germany.