If you run a technical group like an OWASP Local Chapter then how about running a ZAP Hackathon?
You can do it all on your own but if you reach out to the ZAP Team (either directly or via the User Group) then we will be delighted to help you as much as we can.
Setup
Participants who want to change the ZAP code will need to set up a development environment - for that see the ZAP Developer Guide. You might like to suggest that those people set this up prior to the event.
Easy Coding Tasks
Issues labelled good first issue should be a great place to start. If those look too easy then move on to the ones labelled good second issue.
Easy Non-Coding Tasks
Both the documentation and translations are great options for people who do not want to work on technical tasks.
Scan Rules
If you have people with more security experience then helping to improve the scan rules could be an ideal task, and one that could be very collaborative.
If you have some pentesters who are not so interested in coding then they could test ZAP against one of the many deliberately vulnerable web applications).
They could then report any False Positives and False Negatives they find to any people interested in coding, who could then look at improving the existing scan rules or creating new ones.
If you focus on a specific app which we do not currently regularly test ZAP against then you could even work out how to automate ZAP to test and score this app and submit a PR to include it in that set!
Other Options
Please do not feel limited to these suggestions - feel free to reach out and we will be delighted to discuss other options.
ZAP Swag
All of the artwork for the ZAP Swag is (of course) publicly available.
If you would like to have some ZAP stickers (or other similar items) to give out at your ZAP hackathon then please get in touch. We will do our best to get enough posted to you, or find a suitable way for us to reimburse you for any reasonable amount if it is easier for you to get them produced yourself.
We could even create joint artwork if you have your own logo, although be aware we are not that artistic 😉.