Sounds fun. What is your question.
Do you need to do something to the house framing? Are you asking for decorating ideas?
Here's a tutorial for a bed frame, if you the space to just add a frame: https://www.hometalk.com/40947273/customize-your-room-by-building-your-own-bed-frame
Photo would help.
Depends on the size of your alcove. Would a bed fit length or width wise?
For inspiration, here are different variations of built in nook beds: https://www.residencestyle.com/35-amazing-small-space-alcove-beds/