What are good DUPLO projects for around 2 year olds?
And here is my first project: Color towers:

Building towers is probably the first directed building effort that toddlers undertake.  In this project I gave our son (20m) many bricks and built 4x4 monochromatic socles for towers.  I told him that we are building a 'green', 'red', and so on tower.  At first he only got that we are building towers, but I directed him toward using only the color that is already in the socle and after two or three wrongly placed colors he got it.  After a few layers he even asked for specific colors to continue a specific tower.  After the towers were high enough, together we inserted some nooks for figures to stand in.  He did most of the building, and I only occasionally changed the orientation of some 2x4 bricks to make the towers more stable by interlocking.  The visual impression of the result surprised me: