So, you've got (a) coding skills, (b) reasonably regular spare time
and (c) a social conscience
? Contact a bunch of charitable
organisations whose causes you support and offer to do some work for free. This is quite possibly better than a donation
: the amount of money they save not employing a person or company to do the work is probably more than you'd give as a donation; and besides, you're more directly involved and you might even learn something.
HTML and web design skills seem like they'd be the most commonly needed - sites constantly need updating and if you're lucky (and suitably skilled) you might even get a whole new site to get your teeth into - but who knows what else might be useful. Make suggestions, be creative: people are open to all sorts of new ideas once they realise they won't cost them anything.
I haven't actually tried this myself yet, but with a sizable chunk of free time coming up I'm getting together a list of likely candidates. Smaller, more local organisations are at the top of my list, partly because I figure they're more likely to need the help but also because I think it's nice to contribute something to the community.
(By the way, if you're missing (b) or (c) above I'd do something about it!)