Chiang Mai does have a different vibe from Bangkok, but it is still a big, touristy Thai city and not all that different at first glance. The real Thailand(tm) starts if you head up into the mountains for a trek... and if you have only two places to go in Thailand, I'd suggest checking out one of the justly famous beaches instead. Krabi is currently at the top of my heap, with a good mix of dramatic scenery and just enough tourist development (good access and all the mod cons, but no gogo bars and few drunken yobbos). February will also be a very good time to go: weather is just about right, and prices have already fallen back from the end-of-year peaks.