Hi,
can only second what Skyrider said. Obviously you´ll be arriving close to darkness, so the wats (temples) would make no sense. So go and enjoy the nightlife at the destinations Skyrider mentioned.
What location is your hotel? Because at 6 pm most taxi-drivers do not like the idea of going near Silom because of traffic jam. You might need to take the SKytrain though, which is not all that easy for a first-timer.
Whatever you do: Enjoy and play safe
F.R.