Do one portion of a wall in the chalk-board paint. The kids will be able to express their creativity freely, play tic-tac-toe, hangman, play school, etc.! (With the chronic illnesses, maybe a whiteboard instead so they don't have to inhale chalk dust?)
Do another portion of the wall in corkboard, to hang all of their wonderful artwork. Maybe do this one higher up where they can't reach the push-pins until they're old enough. As they get older, you can also hang the schoolwork they're proud of. I'd always wished I'd done a display case too, or special shelving, to have a nice place to put their projects when they bring them home from school: the little clay sculptures, dioramas, etc.
Do lots of cubbies, bookshelves, etc. within their reach so they can learn to put their things away when they're done : )
Have you seen the removable decorative designs? For about $20 you can buy them at most stores now (we got ours at Linens N Things before they closed), stick them on the walls (let the kids help) and as they grow and develop into new interests you can just peel them off and replace them with a design that better suits their current interests. It's the same adhesive as Post-its. My girls have big polka-dots in their bedrooms right now, and had a ball putting them up!
Have fun!