There are lots of places on Orchard Rd. Don't know how many will be open that late though. I have walked there between 8-midnight with no problems. Catching a cab back to the airport you should expect to pay a premium before 6:00 AM. Will cost quite a bit. If you decide not to walk around but stay at the airport overnight there are 2 transit hotels (one in each terminal) that rent in 6 hour blocks. About 56 Singapore dollars for the block. You will not have to clear customs or immigration if you stay there.