Costumepop.com is holding a costume contest. Enter a costume and you can include your site URL. I tested and it is a do-follow link. All submits are manually reviewed and approved so will need a legitimate entry/costume. Here is an example: http://costumepop.com/adult-costumes/zoltar/ Contest: http://costumepop.com/2009-halloween-costume-contest/
Nice find, but it won't be relevant to just about every Web site out there. If the site is relevant to Halloween costumes and parties (and so forth), it'd be worth getting a link from.
Well - perhaps you can find a costume related to your content. Site about cars? Find a car costume. Site about Pets? Submit a pet costume. Site about _______? Submit a _______ costume. Obv include your own tailored description.