Chiang Mai sounds like a good location (they have the zip wire there, waterfalls, spas, elephant trekking, etc.) Chiang Mai is also less busy than places like Bangkok or Phuket.
If you would still like some beaches, however, Krabi is a very relaxed place to go to w/plenty of natural activities.
How much money you will need will largely depend on what kind of hotel you want to stay in. Higher end hotels will easily be north of 150 USD per night, with cheaper hotels and the like costing between 30-100 per night.
http://www.asiawebdirect.com/ has a number of guides for many of the local areas you may be interested in, and you might find some general pricing information for the types of hotels you want to stay in. I would probably budget at least 2-3000 USD for a two week trip.