Paco Park. The Park rental is around 8,000 for affairs from 6am to 6pm, and around 10,000 for affairs from 6pm to 6am.
Their office is at Rizal Park, along TM Kalaw cor. Taft Ave., Manila. It's a green building, it's called National Parks Development Committee/Council.
Their reservation books/calendar open every August. For example, for a wedding for December 2013, you can have it reserved as early as August 2012 (If you are like me who doesn't want to settle for a different date or venue just because someone else got the date/venue earlier than you). To reserve a date, you'll need to pay 20% of the total venue rental, the rest you can pay 1 month prior the wedding.
The person who will assist you is Ms. Cathy Guadalupe. You can reach her at 302-7079/408-7062, office hours is 8-12; 1-5.