A few suggestions from someone who has helped with the last few Cuboree's for my district where we've pulled consistently increasing numbers (650 at the first event in 2006 to just shy of 1,000 last year).
Structured and officially announced is better! :)
Fall is a great time to do this (depending upon weather where you live obviously). Try to plan for right about a month after your begin-of-school-year recruitments have been completed. If your district professionals are going into the schools and doing boy talks, have them push the event. Put the event info in the papers that go home with boys at recruitments and have the packs push them at their recruitment nights and August/September pack meetings.
Our cuboree is generally geared towards beginner campers. We push Scouting as this great opportunity to get outdoors and camp, but boys who join in September sometimes have to wait until the next spring/summer to get that experience. Doing a fall cuboree makes good on that promise scouting gives quickly!
We usually do a Cuboree schedule where families show up Friday afternoon and set up camp. Nothing much on the schedule until a 9pm campfire program. Find someone who is really good at this and have them get boyscouts involved to do skits, songs, etc. That's pretty much it the first night.
The next day starts with breakfast, and then a schedule of station-to-station events. Previous years we've had several different events. Obstacle course, zoo or other wildlife group brings in animals, police, fire department, bb's, archery, tomahawk throwing, tug-of-war, crafts, etc. Last year we did a Bobcat trail where at many of the stations the boys learned one of the Bobcat requirements and as they were able to complete it they got a stamp in a book or a bead or something like that (can't remember for sure). While all this was going on we also run a training session for new leaders (2006-2008 I ran NLE; 2009 we did This is Scouting; in 2010 I'm doing Youth Protection).
From an organization standpoint, we found a sucker…I mean a wonderful and patient lady who spearheaded everything. District professional staff, other area leaders and district volunteers helped with planning and doing. She coordinated the whole thing and during the event itself was free to roam and help where needed (if you've been to national camp school we basically made her a combination of camp and program director). Different units or leaders were put in charge of specific areas. For instance I volunteer to make sure the leader training portion works and my pack handles the Friday night campfire program. Another area boy scout troop runs the kitchen and also sells snacks during Friday evening and during the day on saturday. Other areas that are handled by outside groups (like the zoo or police) provide their own program. There's a trade off for each. For the outside groups, for many the trade off is payment, that's part of the budget. For the kitchen, the troop that runs concessions gets to keep the profits made as a unit fundraiser. For the campfire program, my pack gets to use the castle bunkhouse on site so none of our families have to worry about pitching tents.
If you're only expecting 30 people, 3 groups of 10 would be fine. I'd suggest trying to get a certified rangemaster and do archery and bb's because as a district event you can run ranges with certified personnel. A group of 10 spending an hour at a station would be more than enough time for the kids to get through the safety and procedural rules, shoot and learn enough extra info to get both the beltloop and pin.
Hope some of this helps you. Feel free to message me if you want more information, if your total sizes coming are going to be sub-100 folks, I can probably share a lot of my pack campout planning stuff as we generally get 60-100 people at those and they're pretty structured too (I'm all about organization and structure).