My nephew is 20 months old and significantly delayed but without diagnosis yet. SIL has asked for things that will help with his development. He is not able to stand properly yet (he has bilateral talipes) but can commando crawl and climb. He loves balls and has a large 4 ball marble run type thing which he absolutely adores.
No speech but can sign some things.
Anything that stimulates development of physical, social and educational skills and they're not fussy about it being plastic, lit up and noisy if necessary!