I am often asked about specific events and am glad I can point to so many alternatives these days. But there are so many and hard to keep up to date or even remember!
At a teacher’s workshop yesterday, NHB introduced a GCal-like heritage calendar with email, rss and ical subscription options. And an easy to remember URL: heritagecalendar.sg/
The nature equivalent is wildsingaporehappenings.blogspot.com/ which is maintained by Ria Tan.
Nice that there are easy to point to pages now for all of this.
Otterman speaks... 
I also keep update a GCal version of Ria’s calendar at my blog naturespies.blogspot.com