I need to create log entries about different actions in my system. For example if the user saves a model, deletes one etc. Where should this kind of log go?

I know I can write custom data to the Event log but this is not good, as the Event log seems better left for errors and such.

Logging supports different log levels: http://octobercms.com/docs/services/error-log#logging

You can also consider writing your own logging driver/tables and use these to store the informations, if you dont want your error log to be too clustered with that.