What are your ideas for this interior brick planter?

I would take it out and maybe put in a closet .

This would be fun you can even DIY it https://www.youtube.com/watch?v=oUqzIkjgzMY

My vote is for a mini mud room. Can be done up real cute for minimal money. Try and remove the bricks in one piece to be upcycled in the garden later on.

Some coat hooks a little bench to sit or place you bag, muddy shoe rack.

Make it a mud room for boots and jackets

Cut plywood to size, cover it with foam to make a seat out of it, use the space for storage...small garden tools perhaps? Or shoes? or Cleaning supplies. For those with green thumbs, plants maybe? Make sure there are some UV lights to support the plants. Have a mirror installed on the wall, make shelves from the bottom up. Could be a storage.

I would put a water feature and tall green plants in it

I would pull it out. Try to save the bricks to reuse elsewhere. If they need storage build a closet in the space. Otherwise you could paint it white, cut and paint a piece of wood, attach it to the top, cap it with trim, and use it as a bench

seat. Then place a mirror and shelf with coat hooks to hang above it. You could also cover it with MDF and raise it to the height of an entry table. It’s doesn’t seem to get enough light for any living plants. Best of luck to your son on his future projects in his new home.

I would definitely take it out and leave the place empty. There are several changes you could make in your home to give it a fresh look. You could hang interesting pendant lights, adopt an open plan design, add some eye-catching shine, install some printed splashbacks like the ones from https://www.diysplashbacks.co.uk/printed-splashbacks, decorate with large plants, dress-up your walls or elevate with white woodwork. I would also invest in art displays, but of course the budget is an important factor. At the end of the day, you can always leave it as it is and just go on with it.

Fountain?

Take it out and use the space as a coat and shoe store!