Testing new methods for "praising" my son without saying "good job"

I've been talking with a few friends about our efforts to not over-praise our kids. We're trying to suppress our "good job" reaction (see Books by Alfie Kohn for discussion and comments on rewards) to things our kids are doing and it's more difficult than we anticipated. The other night I had the opportunity to try to use conversation (interest in what my son was doing) as an alternative to praise (it seems so simple when I write it). He was playing with one of his favorite toys, Kid K'nexicon, and had just added rods to every possible space on a radial piece (there were 8 rods around and two rods sticking out of the ends). Here is a little of our conversation...

My son called loudly from his room.
H (him): Mommy, mommy come look at what I built!
M (me): trying not to say "that's great": Wow, you used a lot of the rods. It looks like you attached rods to every hole.
H: It's the biggest thing I('ve) ever made.
M: It's really big. Are you going to make it bigger? [commenting on the piece in his hand].
H: Yes! Look what I can do. I'm giving it mouse eyes.
M: Wow. It got bigger.
H: I'm going to give it more eyes. [He attaches two more eyes into the holes for the mouse's ears] See. One two three four, four eyes. It's the biggest mouse I('ve) ever built.
M: It's a really big mouse, look at all those eyes...

The conversation continued as he added more pieces and moved his creation around. He was so excited. At times I found myself having to pause and think of what to say next. I was proud of him for figuring out how to build something in three dimensions and the more I talked with him about it, the more I realized that talking about it was what he wanted. I have to admit that since we've had our second, I haven't spent as much time talking to our first about things he wants to talk about. We seem to spend more time talking about how we have to wash our hands after using the bathroom or that he can't push, hit, knockdown, or scream at his little brother. I hope I can make the time to have more conversations with him like this one.

BTW: The mouse head was from the Kid K'Nex Story Pals: The Lion & The Mouseicon set and the other components were from several additional Kid K'nexicon sets.