I had a room once and found it important not to make it a “teaching space” it was an “escape” place where kids (and more often than not adults) could come to let off steam.
It started with a nice big hexagonal table and chairs
Other than that, some things that worked were…
Keep the colours neutral and not to stimulating.
Have some quiet music.
Beanbags and/or a small sofa (parents are always changing their sofas and you might get this for free!)
LEGO – lots.
Mindfullness colouring books / photocopiable – there is not enough colouring in school and children need it for downtime/engagement.
A large biscuit tin (with biscuits in it) – this can be very motivating!
Try and get a sink in it so that you can make drinks.
Some noise cancelling headphones / ear defenders.
A light tube.
Hot water bottles or microwavable heat pads (for period cramps etc.) (this assumes you have a kettle or microwave so add them to the lis!)
Have fun with it.