Could you post a wider shot from farther away so we can get some context? It's hard to give advice with just this pic. Thank you!

Go to Costco buy bigger tree like 5to6ft tall or a book shelve

You can do different things with it. My first approach is usually 'function'. If I feel the space can be used for a good purpose, ex: bookshelf if you could use more storage or create sort of a desk with some overhead cabinets. You can use the desk to keep some nick-knacks and the cabinet to hide clutter. If you cannot think of any major function, then simply build shelves in this corner to display pictures, art, put some baskets, etc. But the above comment is right, it is important to see this corner in relation to the rest of the room.

Depends what you need like ATP Designs said. Desk maybe. Place to hang jackets take off boots, add a bench and hooks. Or just shelfs. I wish I had a little nook like this.

I love the idea of floating bookshelves!