I think the niche is probably designed for a hutch, but if you don't want to do that, an appropriately-sized sideboard would look nice and provide extra storage, with a painting above.

You didn't say what is beyond the dining room. The kitchen? How much space you need behind the dining chair (in the 2nd pic) depends on where that walkway leads and whether people will be walking through there while someone is seated at the end of the table. My impression is that you could go longer with the table by up to a foot and still pretty easily pass behind someone seated there.

Of the two tables, I prefer the walnut. I think either would look nice with your floor.

I really don't care for the wood paneling on the back of the fireplace. It looks very haphazard, rather than like a thoughtful design element. If there is tile (stone?) underneath, I would think about removing the paneling. If you think the expanse of tile is too much, there are more aesthetically pleasing ways to break it up.

Thanks for the tips. I don't think I can remove the wood paneling since it seems to be placed under the stone. I could always put some artwork on that wooden ledge to hide the wood.

This is whats behind the table (entryway to the kitchen) I wonder if I don't put anything on the niche if that will look too weird and just push the table closer to the wall. Also, does anyone have recommendations for chairs?

I agree the walnut table. You have plenty of room to walk around with guests seated. I prefer skinny tables so the conversation circle of those across the table is larger = more fun! The ledge could use a collection of tall glass would be pretty. It will make the ledge feel intentional. And yes you could lean a piece of art with the glass pieces on each side. No you cannot push the table closer to make the niche go away. You need to make it feel intentional. The best priced move would be as you suggested a vey big piece of art. Like an 1" or less around the art and long to almost fill up the back of the niche. I'm a fan of MD-Canvas they have $70 off right now - a possible option since they have big pieces.

You need more color in your house. The wall in your living room with the art could be a paint color. Draw from the stone in the dining room. Pull a dusty peach or mauve color. The niche in your dining room could also have a soft color only on the back wall. Chairs best not to be wood and could be a color. Your place needs some color. Enjoy lots of fun items you have!

Thanks for all the feedback Suzette. I like the idea of the big canvas :) What do you mean by no wood chairs that are colorful? Do you have an example?

Here are some upholstered mid-century dining chairs in non-neutrals.

https://www.houzz.com/products/gdf-studio-molly-mid-century-modern-dining-chairs-mint-natural-walnut-set-of-2-prvw-vr~112870734

https://www.houzz.com/products/gdf-studio-serra-mid-century-fabric-dining-chairs-green-dark-brown-set-of-2-prvw-vr~101894468

I like the darker table also but don't think you should go much bigger. Generally you want easy flow around the table to allow people to walk by and also allow for someone to get up and push the chair away comfortably. Think a thin console table like those shown above would be great as server and decorative element- perfect for a nook. Agree with color comment and I'd go one step further and say you need to add a little "softness" as wood, stone and white can be hard and appear cold. An area rug, colorful artwork and accessories both on the ledge and the walls will do wonders!