The beauty of a BioBlitz is that you really only need people, a location, and iNaturalist. You can make the event itself as simple or as complex as you like. Some simple elements to include are:
Welcome and iNaturalist tutorial
Welcome your guests, acknowledge country, introduce your volunteers and run through using iNaturalist on the day
Searching time
A good set-up is for participants to spend time at different 'stations' with a local expert at each one. For example, a section of track on site might be dedicated to looking for birds, another section for plants etc. You can suggest participants rotate stations at regular intervals, and ask volunteer leaders to help make the swap-overs go smoothly. Another option is to have participants spread out on-site with local expert guides wandering around to answer questions or point out unique wildlife.
Wrap-up and social time
Bring everyone back together at the end to chat about what they saw, share highlights of the day, and to share details of upcoming events. You might have a picnic or BBQ after so folks can stay and chat, and get to know one another.