I put this tray together for Baby Bean since he is 100% focused on crawling/slithering at the moment. Just three balls. The smallest is a bean bag type ball, a woollen ball and inflatable globe. The globe is by far his favourite and he has completed several lengths batting it and slithering after it.
To facilitate his crawling further we have cleared our dining room table to one side so Baby bean has the length of our house – the lounge and dining room to move around in. I also try to remove his socks and ensure he is in comfy clothes – no awkward buttons in the way.