Hi there @MakersRanders,
Thanks for posting this great question to the Community! It would be interesting to hear about a native appointment system integrated into Klaviyo.
To start, I have seen customers implement some sort of time keeping/appointment scheduler within their sites + send that data over to Klaviyo. The method to do this relies on more technical skills to get it going as it requires use and understanding of our APIs. If possible, I would suggest reaching out to your dev team and see if this is an explorable option for you all.
Secondly, there is a way to attach some sort of property to customers so you can track these Booked appointments. In our documentation regarding a Guide to Properties, the section of adding a custom property shows three methods of adding a property.
You could definitely have an excel sheet of your customers that have booked, attach a property, and then import that back into Klaviyo. Or you could manually add these properties within a profile. It would depend on which workflow you deem most fit and easiest for your personal use.
Hope this helped!
Alex