Activity log choices - how to add more dropdowns?

My Sales team do demonstrations of our software, we are wanting to track how many demonstrations they do per time period, but would also like to also build a report that shows how many demonstrations we have done for a potential customer. 

I thought of adding Demonstration (Demo) to the Log Activity dropdown, but I don’t see that ability, which leads me to think those are just standard selections only. 
 

Ideas how to track demonstrations separate from other meetings we have with them, then I think I can pull it into my activity insight (hopefully).