I was there a couple times riding, south and north. But years ago so I have no current info. I was there mid 90's. I met David Unkovich (sp?) I think he still runs a tour/rental company. I sat down with David and he had all kinds of maps and drew out some routes for me. Great guy. At the time he was just beginning his touring business, wasn't renting bikes then at all.
Check out the Golden Triangle Riders link I shared above, that's David's baby.
About David Unkovich
The South, for me, was too crowded. I loved Chaing Mai, different world. But even back then, the bars were hopping, but AIDS was rampant.
The south offers a very different experience compared to the north. In the south things can be more crowded depending on where you go, but it's where folks go to hit the beach. Heading north is all jungle and highland, not so many people head to Thailand for that and so things are quite a bit more chill. I can tell you thought that having visited Chiang Mai a few times over the last eight years it's getting busier, thus my suggestion to hit Chiang Mai, rent a bike and head on to Chiang Rai, Mae Hong Son, Pai, etc. In any case it's all going to be very relaxing with lots of riding to be had.
As suggested, hook up with Euro travelers at Euro guest houses. Tons of Germans used to go there (I rode with some) no idea about now. Best time is November/December. Rainy season is over, bit cooler, most of the mud is dried up. Impossible to ride in Thai mud, IMO.
+1 on November and December, also January, although in the end of December and into January you start to hit the high tourist season, whether that's an issue for the OP or not (shouldn't be, makes it easier to hook up with a cute Euro girl
).
Bangkok is tough, but the temples and sights are worth seeing. Hire a guide. Bicycle may be the best way to get around? Hard to get anywhere fast. Be cool, have a ball.
I personally love Bangkok, but I'm into loud dirty crowded cities so there you go. Wat Pho, Wat Arun, Royal Palace and of course Lumphini or Ratchadamnoen Stadiums for some Muay Thai are all must see locations and easily visited in a few days (gotta see the kickboxing on Friday/Saturday night to watch the best matches).
I haven't tried it myself but Bangkok actually now has a subway system, and I've always found the skytrain extremely fast and easy to use (although limited in overall reach). Aside from that, quite seriously, take a taxi and enjoy the air conditioning. For fun at least once take a tuk tuk, you'll have to haggle and unless you speak a bit of Thai (pom phut pasa Thai nit noy! - "I speak a little Thai") you'll pay too much, but it's one of those "must do" experiences.
The Thai people are so friendly and welcoming ... just wonderful and warm everywhere you go. Do try to learn a few phrases ... and smile!
+1000000000000
It's called the "Land of Smiles" for a reason. I should meet up with Turbongr and over a few beers teach him some of the basic phrases ("thank you", "hello", "goodbye", "I don't like to eat spicy" or "I do like to eat spicy", etc. - lol).