One of the things that people seem to have the most trouble with when starting a new AIS project is actually coming up with a niche to work in (whether that work is an information product, Adsense site, affiliate site or anything else…)
A great way to get some ideas is to go outside (no, really ) and browse the magazine section of your local newsagent.
Publishing a print magazine is obviously more expensive than publishing online, so if there is a niche with a magazine dedicated to it, you can be pretty sure that there's money in it – the magazine will only be published if people are expected to read it, and enough advertisers can be found.
When you've found a niche you think you'd be interested in (let's say cycling), buy a copy of the magazine and take a look at what products are being advertised inside – this is great for finding sub-niches for you to exploit (bikes, clothing, route maps, insurance etc…) You can even check whether the advertisers themselves have affiliate schemes set up for you to promote their products!
Of course, you can also have a look at different magazines online – a good site to find them is mediauk.com. This has the disadvantage of not being able to read inside the magazine, but is a good starting point.