You can’t go wrong with stuffed animals. Make sure there aren’t any choking hazards. Anything colorful and chew tolerant is good too.
Edit: And makes noise too is good.
I’d go with a nice activity quilt. This is the age where babies need lots and lots of tummy time. They are learning to roll over and are preparing to crawl. An activity quilt will provide hand and eye coordination and an interesting place to play while stretching and wiggling little legs and arms.
The cardboard roll from the toilet paper, the keys to your house, the box of crackers after the crackers are eaten, filled with cheerios and resealed, tennis balls, and your wife’s favorite earrings, preferably still on her ears.
My children all loved this type of activity centre. It stimulates them mentally and develops eye to hand coordination. They also liked the board type activity centre with bells and balls that move or beep when they bang them. Colour, noise and interactivity.
An activity blanket is good. That’s a blanket with lots of different textures and things to play with. My daughter’s busy blanket had a little mirror that she loved back then. It gives them something to do on their tummies. Lots of babies don’t like tummy time and need something interesting to distract them.