Hi OP,
Congratulations I did exactly the same before both the children were born.
These the items our friend who is a children's physiologist specializing in early education.
PLay Gym we loved the Fisherprice jungle one- Developmental skills encouraged: tummy play, rolling, midline play on back or in sitting, reaching, grasping, hand-eye coordination, depth perception
Plastic Links I think we got there in ELC - Developmental skills encouraged: reaching, grasping, hand-eye coordination, oral exploration, oral discrimination, tactile exploration
Mirror we loved our lamaze one: Developmental skills encouraged: tummy play, head control, social emotional skills, self-awareness.
Bath Toys little squishy ones ideally: Developmental skills encouraged: visual tracking, reaching, grasping, midline play, transferring between hands, banging together, oral exploration, oral discrimination, hand strengthening
Set of rattle balls:Developmental skills encouraged: reaching, grasping, midline play, transferring between hands, banging together, sound localization, sound discrimination, object permanence, visual tracking, joint attention, pointing when rolls away, crawling
Ball Drop toy we had a few the elephant one is brilliant. Developmental skills encouraged: tummy play, grasping, reaching, releasing, hand-eye coordination, depth perception, midline play, transferring between hands, banging together, sound localization, visual tracking, sitting, standing, cruising, squatting, kneeling, walking
Board books - Developmental skills encouraged: tummy play, midline play, sitting, reaching, visual scanning, turning pages, joint attention, pointing.
Soft, crinkly blocks again we loved our lamaze set - Developmental skills encouraged: midline play, tummy play, sitting, reaching, grasping, transferring between hands, releasing, hand-eye coordination, cause and effect, object permanence, oral exploration, sound localization, sound discrimination, depth perception
Ring stacker - Developmental skills encouraged: tummy play, midline play, transferring between hands, banging together, reaching, grasping, hand-eye coordination, depth perception, cause and effect, beginning understanding of size
Play table with removable legs - Developmental skills encouraged: tummy play, reaching, midline play, cause and effect, sitting, kneeling, standing, squatting, pulling to stand, cruising, turning pages
Approximate age relevant: 4-12 months and beyond
Baby tool bench - Developmental skills encouraged: tummy play, midline play, reaching, cause and effect, sitting, kneeling, sound discrimination
Approximate age relevant: 4-12 months and beyond
Push toy for sitting, standing, and walking - Developmental skills encouraged: sitting, reaching, cause and effect, kneeling, squatting, standing
these are what we started with for the first few months then added items as we needed them.