Guest speakers (depends on the area. MAF people, DOC, fish 'experts', lanscapers, tank builders etc)
Have demonstrations on building tanks, landscape design, basic "how to" set up a tank, making light hoods or tank lids and stuff.
Have everyone bring samples of tank and tap water plus any pH kits they have and see if all read the same and what range of pH tanks and taps have.
Go on trips to lfs (if you have a selection to choose from), rock pool areas, fish hunts, driftwood collecting.
Have BBQs and social outings
Have a sales table, auction or swap meet