What a loaded question!! eBooks, definitely, but then you still need to find a profitable niche, research keywords, etc. And, I hate to burst anyone's bubble, but it's not likely that you'll ever just "grow your site in search engines and relax seeing my money raising." There is A LOT of work involved, and to stay successful, you've got to keep working.
You can sell ebooks, online videos in any niche that is profitable. Like Making money online, health related products, Sports related products, Video games etc. However, the key thing is to do the market research so You know that what You are selling is worth the trouble or not. You could have a great quality product but if know one cares about what You are selling they will not buy it even if it is in a good market! So go do some research before You decide on what to sell!