What about a 'portrait' workshop? I think everyone wants to be able to take better pics of their nearest and dearest and this could give them some pointers - and perhaps the opportunity to use some kit that they haven't used before e.g. lights or reflectors.
If you wanted to up the takings part of the deal could be a that they'd each have a decent self-portrait on file at the end of the session. Doesn't have to be a hard copy - if they are bringing a camera then they'll have a memory card with them.
What is the charity for ... this might help with thinking of a theme perhaps?
OK, perhaps not the lights thing then, having seen your next thread - but maybe 'urban' or 'docu-style' portraits.
