Yes, you'll have time for some hawker centres; make sure to get your onward boarding pass in advance though and ensure you have identified your transport back to the airport in advance...i.e. near to an MRT station or taxi rank.
I almost got caught out on a Friday night where the taxi queue from the beach ended up being 30-40 minutes deep! Got around that back ordering a taxi by phone but it was close!
I doubt 11:00 - 12:00am will be too busy though.
Not sure when the first Heritage Tour starts though; but they do include transport normally and a time buffer for delays.