Tinker Toys, Lincoln Logs, maybe an Erector set when they get old enough not to cut their fingers (which for me was about 27), Legos to keep Mom and Dad from walking around barefoot, and plenty of cardboard rolls from Christmas paper to have sword-fights with...
Lego Introduces New Sharper Bricks That Instantly Kill You When You Step On Them
http://www.freerepublic.com/focus/f-chat/3663373/posts
We used curtain rods for sword fights and sniper shots