You can build a mobile app, submit to the app stores and see how it sells. Track your sales, dive into the numbers and find out what works and what doesn't. Does a good design help? What key words help your app get found? What about your UI? Do people love it? Hate it? Why?
You can build a static website and attach Google Analytics and see where your traffic comes from, how is your SEO working? What mobile users are on your site? What pages are they viewing? What's your conversion rate?
Build an e-commerce website and see what sells. Does the position of the items matter? What colors are people buying? What are your buyers preferences? Are your price points too high? Too low?
There's so many cool things you can learn when you start to get interested in marketing. If you're hacker, you can have a lot of fun and learn a ton of stuff just by creating simple things and seeing how the public/users react and use it. It really is completely fascinating.