I've done some goofy apps just for fun and released them for free, but I went the advert route. Check out AdWhirl (
www.adwhirl.com) - this has benefits over just implementing iAd because you can have ads from multiple sources, and you can set up your own ads (AdWhirl calls them House Ads) that cost nothing and advertise your other apps. You get to set what ratio of time is spent looking at which type of ads so, if you wanted to, you could only show House Ads, and you get the click-though data live as well.
For my purposes, it's a nice mix of some ad revenue and also self promotion, if you can spare the screen real estate for the ads.
