I had no plan of taking a double decker to/from Changi. Take the MRT into town to some interchange. Take a double decker from there for a short ride. Maybe to Orchard or Little India. Hop off, eat lunch. Find way back to an MRT station and head back to Changi.

If someone had a list of routes on which the double deckers regularly ply, i can try and work a plan around that.

Zoo/Bird park is an after thought and not a must. No point trying to squeeze too much into 12 hours.