I think you should go for selling service or mobile website. If you make a mobile website then you will get huge traffic and if you submit you site to any pay per view or pay per impression network then you can make money from there.
On the other hand if you go for selling service like SEO, SMM, Forum Posting, Social Bookmarking then you can also make money just doing SEO. SMM, Forum posting, Social Bookmarking for your client's website.
I think both are profitable but you need to be dedicated to what you are doing, thats it.