Zoho Planner offers some formatting options. You can put these options to good use while making your To-Dos lists and Notes. Say for example, you can use Bold text for highlighting a particular To Do to convey that it is more important to you than others.
The help page for knowing the various formatting options can be accessed by clicking on the ‘Text Formatting‘ link at the bottom-left of your Zoho Planner page. Also, this Zoho Wiki page helps illustrate the various options as well.