Don't sell yourself short, all of these options could easily bring in more than $50 per day.
Think about your current skill set versus what you would like to learn that would help your career and future earning potential. Balance those two items to help you choose.
If you have an interest in all three develop all three ideas over time.
For the short term I would pick the area that you are most skilled in so you have the best chance for success.
Digital content probably has the best chance for success depending on your knowledge base and target audience. An e-book could be put together fairly quickly. Pick something you are passionate about and put one on the market and see how it goes.
Web Apps are great for reoccurring revenue, the software as a service model is one of the best business models out there. Find a problem and solve it with a web application.
Apps would be the most difficult to break into and have a money maker the first time. It's definitely a high demand skill and something you should develop for the future if you have an interest.