Thanks Maquenzie for your advice! That's very helpful especially when you talk about the struggle vs. interest.
Actually, my baby uses this object permanence box :
http://www.absorbentminds.co.uk/acatalog/info_T_40_02_20.html It is a versatile box that will become eventually a shape sorter. For the moment, I am just using the lid for the ball and I removed the drawer so it is like an object permanence box but without a tray. I don't know if it is too easy so I should "upgrade" it somehow (add the drawer or change the lid??).
For modeling, I spend my day tidying up his room,
, although I just put a few toys at a time.