I don't know how these activities would go over in a classroom setting but:
showing a range of images and how they appear to people with color blindness (a bonus here is that you are likely to have a colour blind student given it is present in 1 in 10 males). Interesting things you could show are animal camoflage patterns that only work in black and white, or those dot patterns for color-blindness testing, and standard warning/road signs. You can run images and websites through VisCheck to simulate color-blindness and demonstrate that some images (e.g. the dot test) lose meaning, while others (e.g. warning signs) are specifically designed not to. I believe the VisCheck program has trouble with modern CSS designs but you could run it on an image of the webpage.
getting students to use a screen reader e.g. JAWS to complete simple tasks on the web with the monitor turned off to simulate total blindness. I'd guess total failure for these tasks, as there's a bit of a learning curve with these things.
viewing website with tools like Lynx gives some idea of what blind users will hear. There used to be an online simulator but it blocks access to most sites because of abuse. It's available for most platforms though. Maybe the firefox add-on Fangs which seeks to provide a textual equivalent to what Jaws users would hear would be interesting. (It may be too advanced, but pointing out that what a blind web surfer hears is the same as what Google indexes is an important point).
you could find some DHTML/AJAX sites that require some degree of hand-eye co-ordination (particularly anything with heirarchical pop-up menus) and challange the students to operate the sites with the mouse using their left-hand (or right for left-handers). You should find that most will just about get by with Windows or Mac OS X gui's but that most websites that imitate these GUI conventions will fail miserably for those with poor motor ability.
you could try accessing different sites via a mobile phone or PDA browser. Generally accessibly sites are better for this purpose too.
after going over the basics you might want to get students to rank sites purely by letting them see what a site looks like and perhaps use briefly before ranking them in order of accessability. By choosing some beautiful sites that are accessible and ugly sites that are not you could challenge the assumption that accessible necessarily equals ugly.
Also, Mark Pilgrim's Dive into Accessibility is a great introduction to the topic. It might be a bit much for your students but I'd imagine there's a lot of great material there that could transfer without much work, as the tone is very straightforward and down-to-earth.