>[...] I feel like I could make these ideas work, but it's also super risky. I don't want to spend 1-2 years working on these ideas only to look up and realized I wasted my time and money.
If you can't get comfortable with that risk, you're going to have a very hard time being an entrepreneur.
There's always people who luck into an amazing business and business model, but realistically, you're going to have to put some resources in before you figure out if there's value. Even if it's just mocks and time spent talking to folks (do both those before coding anything though!)
Oh, and yelp is a terrible business -- the coding is straightforward. Your problem there is entirely how do you seed a site with reviews and businesses. Typically, two-sided marketplaces are some of the hardest businesses to create.