Good question. It really depends, what types of products are you looking to sell? There are a variety of print-on-demand apps that you can use to product source. These will allow you to sell a variety of customized products such as clothing, pillows, cups, hats, etc. In this case, an app such as T-Pop or Zakeke Product Customizer.
If there is anything else I can help you with, please let me know.
Please check the recommended Shopify APPS for you.