Content marketing is essentially a strategic approach that focuses on creating and delivering relevant content with the singular aim of adding value and attracting customers. It requires appropriate research, the inclusion of keywords, the addition of links, and generation of consistent content. If your content is thorough, useful, and entertaining, it can easily sway the target audience in your favour!
Because digital marketing has so many options and strategies associated with it, you can get creative and experiment with a variety of marketing tactics on a budget. With digital marketing, you can also use tools like analytics dashboards to monitor the success and ROI of your campaigns more than you could with a traditional promotional content -- such as a billboard or print ad.
Google Trends - A good tool for understanding seasonal content. It also displays results in a more visual way but presents much simpler results than Keyword Planner. It shows interest in keywords and search queries, but doesn't give you the exact number of searches or your organic position. It is definitely worth having with Keyword Planner, but I wouldn't recommend only using Trends.
As for what should your sandbox be, you have a few options. The easiest is to start a blog. That’s what this site was started as: a sandbox for learning content marketing, and it only became my full-time job on accident. A blog is a perfect sandbox for learning digital marketing since you can test almost all of the tactics on it, it’s low-cost, and you’ll improve your writing in the process.