I put together some concept drawings to solicit some feedback from my kids. Here’s one concept with a desk shelf, cork board, and cubby shelf. I had other ideas with no desk just a bean bag and a small table, others with whiteboards on all back sides and the desk across the front, another with a corner desk only. Using sketchup works well, I can rotate, add, subtract, all pretty painlessly, and get real time feedback as to what looks good, and what gets no interest.
I think I’ve decided to go for a wedged half dovetail to connect the rails the posts.
I still have not settled on a good way to attach the end rails to the posts. Right now I’m showing them notched and glued and screwed. Not my first choice but I’m having a hard time coming up with a better answer.