How about google ads, but if you donate you get the version without the google ads? Easy as that really.
You get paid for page impressions too (i.e. how many times a particular ad appears on a site.) Not very much, but for a site with this much traffic it'll add up. I bet you'd make around $400 - $600, easily. If someone actually clicks, you get between .10 - $1.25, depending. I find most clicks average out to around 13 cents. One more thing, you can't actually tell people to click on the ads, else Google considers that agianst the rules. But I would click 3 or 4 per visit.
Not to be a party pooper, but I would NOT buy a t-shirt. I have plenty as it is, and would rather spend/donate money on something I could use. donation would be preferable to me.
Google ads at the bottom of the page. Easy. I'm more than happy to click on links to help this site. I'm also more than happy to donate via PayPal.