National Optical 138-MS Monocular Microscope

This straightforward microscope works great, and I got mine for less than $85 off of eBay, shipping included.  If you find another National 13X series scope, try to get one with the Mechanical Stage, "MS" option since it makes moving things around at higher power A LOT easier!  The light is good, the feel is smooth, and until you want to get real serious and move into a digital scope or a really high powered stereo scope , this one will do beautifully.  They say it is used in high schools and I can believe is since it is built very ruggedly.  There's a clutch mechanism that prevents stripping when you crank the elevator too far up or down, and it's heft prevents it from being knocked over.  The light works really well too, and I was able to focus it to my poor vision without any trouble.  This is a good scope to get to look at anything around the house up real close, and will probably surprise your kids with what you see.  You can probably spend a lot more on a used scope, but the functionality you get will likely only be used rarely.  I was surprised at how much we use the 10X range, simply seeing things that you can't see with the naked eye but are not microscopic (like details on coins).