Bangkok is quite a city, I knew it was big but hadn't realised quite how big until I saw it for myself, you could spend a week there alone to be honest, but one thing I would recommend you should do if you can is book yourself an evening on top of the Banyan Tree hotel, in the Vertigo Bar & Restaurant ...I did it last year and it was fantastic. It is quite expensive up there though, with a birds eye view of Bangkok, not your usual Thai prices, I recommend the 400g Australian Wagyu steak, lovely ...and not too pricey, they did have some special Japanese steak that was £130 mind

..also the cheapest wine they have is £40 a bottle up there, but they do have some nice beers, it was the only place I found German wheat beer in Thailand.
Also, in Bangkok, if anyone mentions the Pat Pong 'night market' ...don't bother, unless you want to walk around a market full of complete crap and fight off guys trying to sell you 'Sexy Show!' in the many 'clubs' that line this area, it's a notoriously seedy place where mostly foreighners go for sex, but they do have a market there ...I did not know this until I went there on the guides suggestion though. That's what you get for going on holiday with women though, they hear the word 'market' and go ooo shops lets go, only it wasn't exactly what they expected
As for other places to see, I highly recommend Chiang Rai in the far north, and the Golden Triangle trip, you can see Thailand, Burma and Laos all from the same spot, out across the Mekong. I took a boat trip to Don Xao island which is part of Laos and was lucky enough to be able to cross the border at Maisai (I think that was the name) into the Republic of Myanmar (Burma), I would not recommend Burma if you a super sensitive type though, some of what you will likely see there may well upset you, Burma is not like Thailand at all, or Laos for that matter ...and you may very well get followed by the military, I would not recommend you take your passport and any important travel documents into Burma, a good guide will know this and what to do, you can get the papers sorted on the Thailand side of the border and leave your passport with them for safe keeping, also, wathc your pockets/bags (that goes for Bangkok too, but especially in Burma).
I went to this beautiful and beguiling little place called Chiang Saen in Northern Thailand, right on the Mekong, had some tea and some local 'nibbles' at the most beautiful little village green in the shadow of the ancient temple of Wat Pa Sak (I think) and watched some young lads dancing with fire (one of which managed to set his trousers on fire and was promptly jumped on by about 3 others to put him out, much to the merriment of the largely Thai observers).
I also went elephant riding in Chiang Rai province (from a Karen hill tribe village actually), which I highly recommend as an experience worth having, just keep an eye for some of the very large spiders that can end up landing in your damn Howdah when you brush into the trees (which you will as you end up being very high up indeed when you're in the howdah on the elephants back).
I haven't been to Chiang Mai but that comes highly recommended as well, there is also a boat trip you can do from Chiang Saen up the Mekong to Jin Hong in the Yunnan province of China (long trip though, about 15 hours I think).
I also highly recommend you take the time to see Wat Rong Khun, also known as the White Temple, it's not far outside of Chiang Rai city, you have to see it to believe it really, it's just stunning.
I visited quite a few 'hill tribe' villages and people in Northern Thailand, that's worth doing too if you have any interest in that sort of thing.
All in all Thailand is a great place, there is a lot to do and see and really you could go on several 2 week holidays there and only scratch the surface. I have never been south of Bangkok, there is a whole lot to see down there too.