Building a DIY laser home security system is easier than you think, and the beauty of the finished product is that it can be configured to cover the area as needed. This gives you an advantage over some other systems in that it is tailor made to fit your property.
By using a laser pointer with a decent output you can mount as many mirrors as you need to cover all of the entry points to a given area. The only requirement is that you make the beam hit the target when finished routing it around the protected area.
As usual when working with lasers of any kind please exercise caution in pointing the device. Also, if you are not comfortable with building the project or are not sure of your actions please do not take any chances.
Kaleidoscopes are fun and entertaining for young and adults alike. It can be fun to watch the shapes appear, distort, and disappear, with our brain imagining the shapes to be more than they are. This would be a great project to share with a kid, just be certain that any cutting instruments are used carefully. 
Herbie the Mousebot is a very straightforward but rewarding project to build. The mousebot comes in a package with the parts you need, and the assembly should not be difficult. The end project gives you a quick little robotic rodent that follows a light source. You can make it scamper about by shining a light and moving it directly in front of the little bot.