I think just having cylinders intersect the top of of your geometry is the quickest/easiest way.

However if you're going to be looking inside your lego bricks then it won't work. You won't have round holes inside at the top, you'll only see the inside face of the flat top you've jabbed cylinders into.

Just had a play and made a sub-d proxy of a cube with 8 8-sided poly cylinders booleaned into it. (Why oh why didnt I experiment with a lego cube?)

That is how I would do it if I had to.... Hope someone else helps you, circular holes/depressions into flat geometry seems to be a FAQ for many of us noobs.