My daughter was five last month and we had a party for her and 14 others at home.
We hired a bouncy castle for £65. It was not quite big enough for all the kids so we had some balls and hula hoops which kept them all occupied.
Re. organised games I bought a couple of pinatas from Asda, stuffed them with sweets, and hung them on the washing line for the kids to hit - that went down well.
We also played musical statues which was just about ok with 15 but would be problematical with 30.
If you are doing food it might be an idea to do two serving giving you two groups of 15.
Good luck - I hope that the weather is ok for you.