Repaint the table tennis table with neutral colours to match room.

Thanks, but won't be doing that, so want to tie in red and blue to existing colour scheme

Paint the room white to give it energy. The table is in primary colours and the current room colour doesn’t match. You then want to bring out the mountain colour of the painting in accessories and repeat the red of the table legs. If you want to keep the table tennis table blue and red then you need to embrace that theme to make the room cohesive.

Move the painting to the other wall so it is a reflection of nature outside not competing with table. Add some timber floating shelves to that wall for storage too. Nice box for balls and on back of stair wall a hanger for bats.
If you have space at end go for bar table and stools for spectators to keep them at viewing height. Back up some accessories in the table colours. Even plant pot in the table colour.

Sorry go black on shelves and stools bar table to match windows

You could have low bench seating, I love your hubby’s painting but it’s not getting the justice it deserves on the large wall, it would look beautiful in an entry way, add an abstract to give a vibrancy to the room, fun cushions and add planters near window, keep it simple and stylish to blend with table.

I feel like this article may help you,

https://www.lunarglowdecor.com/home-design-ideas-for-small-living-spaces/ 

Its mostly designing info and tips for small homes but I think you can also use it for a small room like this.

Hope this helps.