If you haven't already sent out the invite to 30 kids, don't do it! I have attended many, many mega parties for kids of all ages over the years and the parent's have seldom said it is worth it. Instead, they have the added stress of constantly trying to top the experience each following birthday. I suppose it depends on your kid and your family dynamic (if you have a big family with lots of kids, then you have no choice) but my son and now my daughter seem to prefer smaller parties where they can do something really special with their friends (last year my daughter opted out of a birthday party other than with the immediate family so she could go up to the mountains and play in the snow with her best friend. She has already put in the hint to take one or two friends to disneyland for her next birthday, which in my opinion is probably still less expensive than feeding, entertaining and gift bagging 30 plus people!)
Here are some ideas that have worked for me and my family in the past.
Pick a simple theme and run with it.
You didn't say the age ranges of the kids, but if they are all around two you can keep it pretty simple.
we did an astronaut theme and made "rocket" packs that the kids decorated and the grown ups did rockets (with alka selzter or something I can't remember it was over a decade ago) - again the kids got to decorate the rockets. Decorations were easy, stars - blow up "earths"
Another year, we did a circus and the kiddie guests all brought a circus animal and did a parade- we set up a bean bag toss, lawn bowling, they did "acrobatics", you could get a clown, etc.
we did an art party where each kid got an artist apron, we set up art supplies, made slime and goop, kid's decorated their own party hats, decorated their own cupcakes.
we did a dinosaur hunt that everybody loved, grownups and kids of all ages. We had a dino egg cake, but we also made chocolate/peanut butter treats that we called dino droppings. We set up dinosaur puzzles, we set up "excavation" tables where the kids all dug for "dinosaur bones," and everyone made dinosaur decorations - pictures of dinos to color, dinosaur foamy stickers, played dinosaur movies, etc.
Of course, these were easy to do because the party size was small - maybe ten to twelve kids and a few grown-ups.