At the very minimum, a "thank you" card would probably be
appreciated.
A book about Singapore, while thoughtful on your part, might not
be particularly useful to the hosts. The food might be a better
idea, though there's the risk Americans might find it a little too
"exotic".
How about a gift certificate from a major retailer? They could
use that during their Christmas shopping.
Although gift cert may be nice but it would show outward how
much the present would had costed you... How about a fruit basket?
Bottle(s) of wine? Something to add on to the night/day that you
spend with their family.