I don't make raised beds, but if I did I would probably try burning the wood's faces for a shou sugi ban style wall. As a bonus it can be really beautiful.
There is a
a thread here on Permies about doing this. Several people mentioned it taking a lot of time and fuel to burn the wood faces. I'd move the wood over a camp fire as I wouldn't want to use fuel. There is also a method where 3 boards are wired together to make a sort of chimney and that is placed over a small fire to get the internal wood faces to char nicely, though seems a bit smoky!