hi, sounds like a great talk ! i don't know how scientific you want to make it, but there are some very good diagrams of how e-ink works which might be interesting to show them. you can find a lot searching on google :
http://images.google.com/images?rlz=...N&hl=en&tab=wi
here is one which can be found on the eink.com site
http://www.eink.com/technology/howitworks.html
it is copyright by the eink company but i am sure they would allow you to use it for educational purposes (it's been used in many different websites already, as you can see in the google results).
another point to make is that ebooks allow easy access to books that might otherwise be hard to find, because they are out of print, but also because (particularly since you say you'll be speaking in ruralish spain, where presumably there are not a lot of big bookstores) you don't need to depend on the stock of paper books which is available in a local bookshop or library, which is often limited even in big cities. this is an amazing ressource to preserve culture and also to discover new authors they might otherwise not be exposed to.
and, you could point out that any of the budding writers in the group (there are always some, at that age, right ?) could easily publish some of their own writing as ebooks and potentially be read by people all over the world. i would talk about feedbooks.com, which has a
spanish language section ; they can find books to read there (for free !), and they could also publish their own works.
let us know how it goes !