Giving Stephen my old Legos (yes, I say "Legos" - sue me) has been great - we've spent a good bit of time together working on them. Typically, Stephen finds one of the instruction books and asks me to make whatever it is. While I'm doing that, he proceeds to make something out of his own imagination out of various other pieces lying around. We've built probably 2/3rds of the space sets I had, though I haven't tried the Galaxy Explorer yet. We've also built a few Town sets - mostly Police vehicles.
This pattern actually leads to some consternation on my part. I've almost always been one to build a Lego set exactly as the directions say, and then leave it that way to play with it. Why? Because the instructions give the Right Way to build it. Sure, I'd occasionally try one of the alternate versions shown on the back of the box (they don't seem to do that anymore), but I always prefer the originally intended design.
Stephen is different. He does like to build according to the instructions (or rather have me do most of the building) and play with that, but then he's very content to take it apart part-way or modify the design in some way. That's just fascinating to me. He also loves to take random parts and put together his "spaceships." Some of them are very creative uses of Lego pieces. He also likes to give them silly made-up names, but they're mostly a variation the same nonsense phrase "ping fong shang" with "robot" and "PDA" usually in there somewhere. He also has made up uses for all the parts he puts on.
My consternation comes in two places. First, his spaceships contain some parts I need when building one the Right Way. He typically wants me to build the set by the instructions and so sacrifices parts from his spaceships. But I still have to find them. Second, building spaceships like that is not the Right Way. It's silly, but I have to get over it, and mostly have. It helps that many of the things he builds are quite creative. I certainly don't want to stifle his creativity, so this has encouraged me to let him go for it and enjoy the cool designs he comes up with.
Perhaps all this is just an excuse to build a bunch of my old Lego sets. It has been lots of fun and Lego time has been good "boy" time for Stephen and me. And the truly great thing about Lego is there really isn't one Right Way to play with them.