If you’re using Project Wonderful and you have a friend who’s roughly on the same level as you, audience-speaking, you could reach an agreement: direct your default Project Wonderful ad to the other guy’s comic for a month or two. Total cost: zero.
Other nifty uses for the default PW ad: merch, donation page, a particularly good storyline. Remember! The space you have in your page is pretty valuable. Some people even call it “real state”. PW ads often go to the default on weekends on many comics, so don’t waste it!