Being sneaky about it and hiding local ads in the banners of other websites is:
a) Rude, and
b) Unlikely to work, since I ignore those banner ads anyway. Even if I saw those ads, I'd be highly suspicious of it (in a "10 local girls are interested in talking to you!" sort of way).
Talk about an opportunity lost. Look at Starbucks' free wifi sign-on page. It's nice to look at. Do the same thing, and it's alright, put some ads on there. I don't mind.