I love that Duplo bricks and standard Lego bricks are compatible. I don't have a ton of Duplo, but I find the extremely useful when I'm building something big and bulky.
I pulled them out just recently when Liam (my son) and I decided to build "Liam Town." Here are a couple of shots:
The main part of this is obviously the train tunnel. In the last picture you can see my son's creation of himself (completely Duplo).
A 2x3 standard Lego brick won't directly connect to a Duplo brick. The standard Lego bricks must be a 2 by ____ (and the blank number needs to be divisible by 2). For example, a 2x4 would work, and so would a 2x6 or 2x8. The nice thing is that once you've covered the Duplo with standard Lego bricks, you can build anything on top of those.
Pretty sweet. We had a lot of fun.
As a side note, you'll notice the doors/windows on our trains are often open. This is the work of little sister, Tigi. She is quite adamant that windows stay open. :)