After using the Mark II for a while, I have found that there are a number of accessories that greatly increase its general utility.

Powerbank - I noted in the details section above that this is a cheap 27AH unit from Amazon. One of its outputs is rated at 5V and 4.5A. So far, I have had no power glitches - even when using a USB SSD drive.

Micro SD extension cable - this is available from Pimoroni and makes it much easier when changing the SD card that the Raspberry Pi boots from. This is very useful as currently Ubuntu does not have the DRM software for ARM architectures necessary to run Netflix, while Raspberry Pi OS does. The picture shows the Mark II running the latest Bookworm release of the RP OS. I velcroed both the end of the extension cable and a spare SD card holder to the inside of the case.

USB speaker - This is a small inexpensive unit available from PiHut that gives surprisingly clear sound.

Bluetooth Mouse - although you can get away with just using the touch screen and the keyboard mouse emulation, both Ubuntu and Raspberry Pi OS are much more usable with a mouse.

GPS USB Unit - available from Amazon and useful with online maps when travelling.

I carry all these accessories in another waterproof case as pictured below - it's a case that I was going to use for Pickle Pi before I decided on the tablet format.

Ideally all these of accessories should have been built into the Mark II case, however, that would have needed a much bigger case and would have been less flexible than the current two case solution.