As a kid, Legos were much more interesting than Duplos: at the smaller scale you could build much more complex things. Until Nora is older, however, we're back to Duplos. Julia recently got a set of Duplo-compatible marble run pieces [1], however, which have been a lot of fun. They stay together better than the wooden marble runs, and offer a lot more potential combinations than the tubular plastic ones.

While the special-purpose pieces are great, a lot of the fun has been in figuring out how to misuse them, or in adapting normal Duplos. Here's something I built yesterday that shows some of this:

For example, it starts with two straight marble pieces, intended to be flat but that can be convinced to sit at a steep angle:

Later it sends the marble through a bunch of pairs of smooth curved normal pieces:

It can also run along the tops of railing pairs, which even lets you do diagonals:

We also have a pair of slides from a playground kit, which are tricky to use well because the ball goes so fast. Here I've cheated a bit by having the ball slow way down when it hits the piece at the end of the slide; much better to do a little drop and keep that speed, if you can use it well:

I made an overly dramatic slow-motion video of it, set to one of the draft tracks from the live album Cecilia and I are Kickstarting:

[1] Something like this. The main downside is that the marble is a chokeable, but it's much easier to keep one small object away from the baby than a large set of small objects. And after each use we can put the marble away out of reach even if we're not cleaning up everything else yet.