A kaizen event should be focused on only one goal plus any safety improvements you can make to the process. Many times I have worked on kaizen events with multiple goals only to see the team lose focus in the middle of the event or the team splitting up to work on them individually.
This is not to say that multiple accomplishments can not be achieved during an event, only that a single focus is better to keep the efforts of the team on track. In my experience, a simple productivity goal works best for a kaizen event. This can be measured in time and/or distance.
It is not uncommon to see a WIP reduction goal on a kaizen event. Any WIP reduction should be a result of kaizen and not specified as a targeted goal to prevent teams from simply removing this inventory without process improvements. With actual process improvements, the need for excess WIP will be removed and the reduction will occur with better, sustainable results.
With one focused goal, many of the brainstorming activities will be improved by increasing the quantity of ideas on one topic area versus multiple ideas across a wide topic range. It forces that team to think deeper into ways to achieve a focused objective.
As for safety improvements, I recommend a small target of 3 to 5 safety improvements per team. This helps emphasize the importance of safety in conjunction with kaizen and may prove challenging in some processes. We should always work at making our workplace safer at every possible opportunity including kaizen events.