Build your own metal detector…
“BFO (beat frequency oscillator) metal detectors use two oscillators, each of which produces a radio frequency. One of these oscillators uses a coil of wire that we call the search loop. The second oscillator uses a much smaller coil of wire, and is usually inside the control box and is called the reference oscillator. By adjusting the oscillators so their frequencies are very nearly the same, the difference between them is made audible as a beat note, this beat note changes slightly when the search loop is moved over or near to a piece of metal.” Link.
You can find these old digital microscopes on eBay for under $20. Here’s a blog that catalogs some of what you can see with them – “Last week, I got myself a toy I’ve wanted to play with for many years — the Intel Play QX3 Digital Microscope. When I was a kid, the old medical microscope my parents bought for me at a garage sale was my favorite toy. I would spend hours putting whatever I could find underneath it… and now, with the digital microscope, I’m doing it again as an adult. I am adding new pictures every day and will continue to do so until I run out of things to look at with the microscope”… [
I think the Double-slit electron diffraction is my favorite – “Robert P. Crease, a member of the philosophy department at the State University of New York at Stony Brook and the historian at Brookhaven National Laboratory, recently asked physicists to nominate the most beautiful experiment of all time. Based on the paper of George Johnson in The New York Times we list below 10 winners of this polling and accompany the short explanations of the physical experiments with computer animations.” 
“A speed trap measures the timing of events or speed of projectiles “This is a very affordable and simple to build speed trap based on the PIC 16C84 microcontroller. The goal was to use as few parts as possible while adding as many useful features as possible.” 
Interesting way to provide localized power sources – “The Toyota Prius – Gasoline-electric hybrid vehicles can serve as a UPS (Uninterruptible Power Supply) for your house. The Prius, unlike the generator, also has a battery that provides instant, UPS-like power, to your house. Careful research has shown that most people would prefer not to go outside to manually pull a cord in the case of a power failure.” Tons of information, photos and how-to info… Thanks RobK123! 
Nice bike – “What does a propulsion engineer do when he wants to experience the power of a rocket without going to space? He simply bolts one to a bike. How It Works – 1. A toggle switch on the battery pack arms the ignition system. 2. The left-thumb button sends power from the battery pack to an igniter on a model-rocket motor inside the rocket engine, vaporizing the roofing-tar fuel so it can burn. 3. The right-thumb button keeps the nitrous oxide flowing (and the rocket lit) as long as it’s pushed. 4. The left brake lever regulates the flow of nitrous and throttles the rocket.”