I would build something I liked doing. I made a couple of blogs in categories I liked a lot- and they are making great profit through AdSense! Pick something you like, and you'll never have to work a day in your life
Create a portfolio of Adsense revenue websites. $ 10, 000 can generate an adsense residual income of more than $40,000 per month in less than 3 years...
Starting websites are always good investments they are like virtual real estate, they are always worth something and their value can go up. I invested my money into niche sites to sell affilaite products and to earn from adsense. Its paid off. Make sure you read up on what exactly you need to do and how to do it before spending any money.