I think trying opt-in Google Ads (and perhaps having a little campaign among site regulars to encourage turning them on) would be a fun experiment . We could have a little link on the donations page that takes people from there to the appropriate section of their preferences, for people considering donating but not willing to go through the hassle of paypal etc (which is probably 10x the number who visit the donations page and actually donate).
To answer a few lingering Google-ad Q's -- if Google thinks it's being gamed by clickthroughs from your site, it can shut off your account. It also retains the ability to freeze your Google-ad account balance if it thinks you or your site visitors have been gaming it, iirc. So, um, don't do it. ^ ^ On the other hand, non-automated clicking on ad links, for fun, is ok; you don't have to intend to buy a product when you follow a link (it's their job to convince you anyway).
+sj+
http://en.wikipedia.org/wiki/Wikipedia:Village_pump#Opt-in_Google-ads.3F