Hi, since activity is dynamically created by users as to-do's, Odoo is definitely couldn't track it based on the to-do's summary because what user fill can be different to each other. What Odoo can track as category is a fields defined inside the Sales Order form view.

The workaround might be using alternative like "tags" field as picture below. here i am defining a tags on this sales order

back to the sales order menu, you can later group using custom group and select the tags. 

Once you group by tags, change the view into kanban. Thus, the SO will be categorized like this.

To save you from hassle to re-filtering things, you can try to save current search as "Activity" and share it by all users.

So, when you go back to Sales order menu view you can just click on "Favorite" and click on "Activity". You can later switch to kanban view again

This is the alternative solution on how you can track your activity in sales order. The whole process will require user whom assigned with activity to manually assign the tags. If you require to make things automatically, development of a custom module is needed. Moreover, the custom development might can be having a better solution and precise rather than this alternative solution.

Hope this helpful, best regards

Altela (altelasoftware.com)