Indie game dev here. While banner ads are the easiest way to monetize, they aren't the most effective.
IAPs with compelling content is really the way to go. But it really depends on the game. If your game is a quick arcade game like flappy bird, its really hard to have any compelling IAPs and ads are probably going to get you the most money. Of course you can also create an IAP to remove the ads, which is always a good idea as some people really hate ads.
Besides banner you can also try full screen interstitial ads in between levels.