That's a lot of people. You would definitely need a function room at a club or hotel. The only outdoor places I can think of is the Peak - Mt. Austin Rd Park but you can't reserve it and Cyberport Podium Garden which is around $18K. The Football Club gym might be a good option, but you need a member to book it for you.
I went to a party there and they had the club's bouncy castle and they rented a fancy one from another vendor. If you do it in the gym, you can play a few relay games, pass the parcel, musical statues, and a arts/crafts table is always a hit.