The first thing you need is a classroom. Or a cub pack, or a youth group, or just about any-place where a bunch of potential geeks gather waiting to be inspired.
Next you need some geek stuff to show them.
Bring something along that interests you and talk about it, demonstrate it, let it loose in the classroom.
Chances are that if you're passionate about it that you'll be able to give a good presentation.
It doesn't have to be expensive (the most expensive session I run costs about AU$4 per geekling).
It's not about what it was made to do, it's about what you can make it do!
Some of the sessions require various programs. To support this I remastered an Ubuntu Live CD.
Simply boot any available PC using the resulting DVD and everything is ready to go.
No DVD? You can convert the botable DVD image into a bootable USB stick using usb-creator-gtk (not written up yet, contact me for help).
If booting off DVD/USB is not an option I implemented (but never got to use) a network-boot based solution. One reasonably grunty machine is all you need to net-boot a room full of PCs.
If all else fails the Live DVD contains windows installers for the programs.
If that fails too then there are some Browser-based activities.