Fifty Dangerous Things (you should let your children do) is full of suggestions to help you overcome the paranoid instincts of modern parenthood.
Backyard Ballistics is a guide to launching things and blowing stuff up. 'Cause you know they want to.
The Book of Totally Irresponsible Science (with a title as bad as this, buying the book is your first step to letting go). Simple stuff, and mostly fun.
Feeling more relaxed? Try Ransome's Pigeon Post for inspiration--read it aloud to the children. It's not a "how to" book, but a story of children who camp by themselves, mine for (what they hope is) gold and try to refine what they find, deal with drought, battle fires, and have wonderful conversations with total strangers. Check out Ransome's other books, too.
PS: Don't forget Forbidden LEGO!!