This is a slimy practice, but I what I wouldn't mind, at all, are ads when I first connect to the AP. Make me watch a video, or let me click through a few pages of ads for local services - if I'm at a hotel, I'm likely from out-of-town and are interested in nearby restaurants and tourist destinations. Show them to me! It's likely that I'm using the internet to look those up anyway.
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.