Talking about birthday party, I did some search few weeks ago and 1 place (outdoor) that you can consider is the Bottle Tree Village/Park.
There are 2 outlets, 1 at Sembawang and another one at Yishun (near Khatib MRT station).
http://www.bottletree.com.sg
The charges for birthday parties are:
(you might need to call them again to confirm as I might got it wrong)
Small size party (less than 60 pax)
- venue: aircon dining hall rental for $200++ (but I am not sure if they partition the area to separate your area from public dining)
- you can use it for 4 hours
- Buffet lunch is charged separtely for $15++ per pax
Medium size party (less than 100 pax - not very sure though)
- venue: only non-aircon area and rental is $400++
- you can use it for 4 hours
- Buffet lunch is charged separtely for $15++ per pax
Activities available at the Bottle Tree Park (Yishun) are:
http://www.bottletree.com.sg/?cat=3