This looks like a primary bedroom, yes? What you put here depends on what activities you want to do in this room and if you would actually spend time in there doing them:

I'm a fan of a chaise lounge for talking on the phone, reading, working on my laptop, petting my dog, and folding laundry.

A window seat

A floor plant and open space for an exercise mat

A collection of plants

A comfortable chair and ottoman

A desk

A vintage trunk with extra linen storage

A round table centered in the space with a vase of flowers and a few coffee table books that you leave open to inspiring pictures

This but longer!

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Office, but if you do any kind of art or craft you can use it for that too.

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Do you think anyone ever sits on this window seat? I don't.

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Depends on what activity would be best for you. A desk? A reading corner with a comfy chaise? A recliner?

I’ve loved these ideas! This house is 7 miles from the Gulf Coast and across the street from Boggy Branch. We will probably turn it into a seasonal rental. It has two master bedrooms down stairs and this huge bedroom upstairs.

I’ve loved these ideas! This house is 7 miles from the Gulf Coast and across the street from Boggy Branch. We will probably turn it into a seasonal rental. It has two master bedrooms down stairs and this huge bedroom upstairs.

If it's going to be a rental, could it be a chair and a half sleeper sofa, so that it would be used as a bed if necessary?

Bookshelves.

Ah, whole different question considering it is a rental. Just buy a long bench to put in the window. Nobody will ever sit on it but will use it for bags, organizing stuff. It will look nice in listing pictures and be practical.

Two comfy chairs, small table.

The reason I suggested more possible sleeping options is because the houses we rented at beach had pullout sofas everywhere, in addition to the bedrooms. The great room had a sleeper sofa, as did the small den and bar area by the pool. Obviously the people who slept on those beds would have no privacy at all, and would have to get up as soon as the first person got up and used the kitchen. We did not have that situation, but the house was set up potentially for that.

With two beds which appear to be fulls rather than twins, this is a multiple sleeping arrangement anyway, I doubt it's going to be an adult couple to each bed, so when more than one person or couple is meant to sleep in a room anyway, two might as well provide options for three or five. The privacy aspect is already not there, so the room is already going to be used a certain way.

You could do a chaise lounge, bench, or even 2 chairs with a side table for a reading nook. Here are some visuals to go with the ideas:

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I'd do either a pretty desk and chair, or a comfortable chair with a side table and a lamp for a reading corner.

What did you end up doing?

With that great natural light, I would just do a desk/table so someone could open up a laptop or even do makeup. I can’t count how many times I’ve been stuck trying to do makeup in a bathroom with no counter space or horrible lighting while traveling. I would love to sit down with a makeup mirror and get my makeup done in under 10 minutes while on vacation which is super easy with good lighting.

Agree with @ShadyWillowFarm.