I did a month in Thailand last September. My first 3 weeks or so I went South and did the islands, then the last week or so before my visa ran out I did Northern Thailand.
Depends waht type of holiday your after but I'd if you've got 3 weeks go south and do the islands. I personally would spend too much time in Bangkok, 3 nights would easily be enough. One place that you can go to which is a good few hours drive from Bangkok is the tiger place. I got some quality photos form there.
Phuket was the first place I went to after Bangkok, I thought it was good at the time but then I jumped on the ferry over to Ko Ph Phi, which I thought was easily better than Phuket. Id' recommend going to Ko Phi Phi.
we went for a day trip to the phi phi islands when staying in krabbi and are planning on going to phi phi don via phuket in november
loved everything about thailand its a great place wished id have gone there years ago when i used to go on lads holidays
After Phi Phi, I went to Krabi town. Didn't like it myself, wish I'd stayed on Phi Phi longer. Then coached it across the mainland to and got the ferry to Ko Samui & then Ko Phangan. I really should have gone round the island more, I did intend to but it's easy just to go out on the piss in the touristy places and get trapped there.
Pattaya is supposed to be, shall we say an interesting place, but never ventured there myself and I guess it's not the kind of place to take the Mrs.
Chang Mai in the North is a pretty cool place but pretty out the way.
Basically I'd defo recommend Ko Phi Phi and Ko Phangan and altough it's a fair old journey I'd try go to the Tiger places I mentioned.