As the name suggests, I'm not a nana (or a mama either...) but I have observed that babies get a tremendous amount of "stuff" - clothes which they grow out of very quickly indeed, lots (and lots and lots) of toys...none of which will be remembered as having been "given my a grandparent in years to come.
I suppose the question is "how well-off are you?" and "how much do you want to spend?"
If you are a bit broke (and aren't we all?) then perhaps you could save a small amount every week - by the time the child is 18 it will have grown into bigger amount for an 18th birthday present.
Perhaps you could find a nice wooden toy chest, or a child's wooden chair (antique or modern) which they in turn can pass on to their own children when the time comes. As someone said upthread, a child's chair can be turned into a chair for soft toys when the child has got too big, and a wooden chest in always useful for storage in later years...
If the baby is a girl. perhaps a nice piece of jewellery which might be worn as they grow up and which will be worn when they are grown up? (I recently had a pair of pearl drop earrings converted into 2 pearl pendants, one of which I put onto a gold chain for the baptism of a child of my acquaintance...she is too young to wear it now, but she will grow into it. (I kept the other one and wear it myself.)
If the baby is a boy that idea is a bit more difficult...