Delegation is important. Only a few tasks are absolutely must-do, but the more help you can get, the better. Assemble a team of trusted friends and colleagues who you know you can rely on to help get the job done.
Sometimes your jammers can also be your volunteers / helpers, if you ask them nicely and the task reasonable, but remember to set expectations and deadlines as well as treating volunteers kindly as their time is valuable. It is fine to hold people accountable for things they have commited to do for you.
Finding a suitable venue, and talking to them to book in the jam
Submitting the application on the GGJ Website, and coordinating with the RO
Checking the set up of tables, chairs, internet, cables and other necessary hardware
Making sure the internet works!
Being in charge of organising security, managing the safe space policy and jammer welfare.
Staying up for, or delegating night shifts, if your location is open 24hrs.
Setting up a space to show the keynote
Doing online promotion for your site
Designing and printing a poster
Organising a small slide presentation for the start of the event
Organising the food and drink
Designing and printing the badges/posters/signage
Technical support for jammers
Taking and sharing photos of the event
... and more ...