if you are going to thailand then you must pay a visit to chiang mai, its an easy 1 hour flight bangkok or if you are feeling adventurous get on the overnight train from bangkok, it takes 12 hours and costs around 30 quid - its a great experiance and when the sunrises over the paddy fields, you feel a world away. the city is much smaller and cleaner than bangkok and there are some great trips to from there for example a typical trip would be a 2 night trek to see the hilltribe villages, you stay in the village, go rafting and ride on elephants - all for about 60 pounds.
Another place i would reccommend is the island of Koh Lanta - if you just want to chill then this is the place to go, nice beaches and friendly atmosphere , you can get a boat from Krabi (which is also nice but more mainstream tourists go there) you can get accomodation when you get there, and you are looking at 10 quid a night for a nice aircondition beach bungalow overlooking the beach. if you are staying in bangkok make it only a few days as its not the nicest of places, make sure you visit thegrand temple and a must is to go to patpong markets (by night) its a real eye opener but if your doing a bit of shopping you can get some great bargains.