I've never flown NW, but I do go to BKK a fair bit.
You might want to consider getting the hotel to arrange a car for you. IIRC the Sheraton Grande Sukhumvit charges about USD30 for a Mercedes and some who meets you in the arrivals hall, shepherds you to the car and concludes with in room check in. Worth it if you are unfamiliar with Bangkok, likely to be jetlagged or fancy a touch of luxury. The Westin offers something similar. Both the Sheraton and Westin are good hotels, well situated for the Skytrain, Subway and the delights of Sukhumvit Rd - which has sone great bars and restaurants.
I am less fond of the Royal Orchid. However for a couple of days it is fine and the riverside location is change to Sukhumvit. Do make sure you do a dinner river tour. IMHO the link below takes you to the best people.
http://www.loynava.com/
A day trip on the river is great as well. Make sure you tick off all the usual tourist sights - the temples never fail to inspire. Bangkok is a fantastic city, though if you haven't visited Asia before it may come as something of a shock - it is big, busy, hot, smelly but lots of fun.
I would agree with comment about using Thai to get to HKT. Domestic business class fares are not much more than economy. If you pick your flight right you can get a international F seat on 747 along with lounge access at both ends. Worth the extra $$$.
Any specific Q's. feel free to PM me.