I used to live in Asia and traveled to Thailand frequently for work (and play).
Milemiles and Done1 covered most of the cautionary stuff, so I'll just add a couple of additional items. If you do get lucky and find a good girl in your first couple of days then get her to go sight seeing with you. Thailand is famous for the GFE so spending the day with you is a standard package. If you get a good girl she will take you to some great restaurants, scenic spots, and generally treat you like a king. If you get a "bad" girl she will want to party, constantly pester you to buy her stuff, try to invite her friends along, etc.
A popular tip for budgeting and also keeping your girl in line is to figure out a daily budget (say 10,000THB). Put that cash in an envelope and use that envelope every time you pay for something (tourist attraction entry, lunch, shopping, gifts for the lady, whatever). At the end of the day you give the envelope to your girl and she keeps whatever is left. They wise up very quickly to help you keep your spending down so that there is more for them. 10,000THB sounds like a lot of money but it's $400 CAD (basically one hour with a mid-priced provider here).
It's been a few years since I was in Thailand but the standard rates used to be 2000THB for long time, and 1000THB for short time. Long time is over night and short time is 2-4hrs. I imagine it's probably double that now but 4000THB is only $150CAD.
Oh, a couple more tips. Get your girl to take you to a traditional Thai massage place (not an upscale spa). You can get a 2hour (non sexual) massage for about $50CAD. This is something you really should experience (trust me). Also since you've never been to Thailand before I would recommend getting a river tour of the temples. There are over 2000 temples in Bangkok alone, and some of them are bigger than any of our cathedrals.
Lastly if you get a chance try to pop up to
Ayutthaya. It's the old capital of Thailand and it's only an hour drive north of Bangkok. Your lady friend can arrange a driver for you for a day trip quite cheap. You can also book a river tour from Bangkok to Ayutthaya for a reasonable price.
Oh and if your gang decides to rent a car be very careful driving there. Not only are you driving on the wrong side of the road, but you are also driving in one of the worst cities on the planet for traffic and driving habits. (When was the last time you had to navigate around an elephant?)
Oh and the locals will love you if you learn just a couple of words of Thai.
Hello is Sawadi Kop
How are you is Sabai dee mai
Thank you is Kop Kune Kop
Just be warned, that Thailand can play with your head and make you want to come back again, and again, and again.
Enjoy your trip!
