This first toy almost doesn't even seem fair. It's almost a gimme -- Duplo Legos. I played with Legos growing up, and our Obed got his first set of Duplos before he was even born (by good friends). We got them out after he was sitting up, but were disappointed to find that all he wanted to do was dump them out of the container, put them back in, and dump them out again (and he wouldn't sit still to put all 70-ish pieces back before putting them back in). Oh and he'd hand pieces to Dad and I and expect US to put the pieces together. I put all but about 10 pieces up and let him play with that manageable number.
Shortly before his second birthday, though (about 20 months), I started noticing that he was actually being able to put the Duplos together. And take them apart again! Score! The old Duplos got brought out and a new set was purchased to give him more to work with. They get played with just about every day. And, just this week, he has sat down and played with them, nearly uninterrupted, for 2 hours! That is unheard of for my son, whose attention span is closer to 10 minutes most of the time.
He doesn't seem to mind when they fall. and he seems to like to build large blocky buildings. His latest trick is that he gets all the 2x2 pieces. We can have anything else we want. But not a single 2x2. Makes us get creative as well.
There's a lot of little pieces. I have another box (Fun with Wheels) and a bag of Duplos that I got at a thrift store, but I do not feel I can bring them out right now because he's got "just enough" for his playing with and I fear more will overwhelm. I hope as he gets older he will need more bricks! In the meantime, I've told myself to stop buying bricks!