I'm in the same boat! I am planning my 5 yr old son's and 2 year old daugher's birthdays in march. will be inviting my son's 36 classmates from preschool, plus friends/relatives. could be between 15-50 kids! wanted something that was flexible re: # of kids - and didn't cost a lot more per child. the local YMCA has a great "gymnastics/sports" party package that was very reasonable (~$150 I think) they do the activites, you bring food, etc.. Unfortunately, my YMCA only does these parties during the week during basketball season - so I'm designing my own on a saturday. I reserved the rec room at the local park ($40/hr,capacity for 50+ people, tables/chairs and kitchen). I didn't want the stress of looking for space on the day (or worrying about rain in the ramadas). The park has a huge playground, and lots of space for other activities. I'm planning some craft activities, sports, musical chairs, a couple of kites, etc. and bringing costco pizzas, veggie tray, snacks. making my own ice cream cone cupcakes. minimal decorating (table clothes, balloons). will order crafts from orientaltrading.com, and clean-up is easy. if the kids want to play longer, we'll just move the leftover food to an outside ramada and stay as long as we want.
good luck!!