Chiang Mai. This is the biggest city in the north and is a must see. It's, hands down, my favorite place in Thailand. You can see elephants (go to Patara Elephant Farm-- they are by far one of the humane and sustainable sanctuaries there are), hike, visit waterfalls (I personally recommend the "Sticky Waterfalls" an hour outside of the city), take cooking classes, enjoy the walking markets, etc. You will definitely want to stay around the Old City and the South Gate, but I recommend The Rim or 99 Gallery Hotel (more budget). If you're looking for cheap accomodations Spicypai Backpackers is a fantastic hostel. The Puripunn Baby Grand is my favorite spot. I'd also suggest heading to Chuang Puak Gate and eating pork from The Pork Lady. She wears a pink cowboy hat and slings the best food on earth for only 30THB.
Some of the best shopping on earth is the North Thai Lanna boutique shops, with beautiful embroidery in bright colors. Chiang Mai has a ton of temples and most of them are overcommercialized and they suck. Wat Sri Suphan is the only temple in Thailand made entirely of silver and is very close to the Old City’s square moat. Wat Chedi Luang is my favorite not necessarily for the architecture but because you have opportunities to chat with monks!
Technically Wat Rong Khun (White Temple) and Black House (which I prefer because its so alt) on Chiang Rai but there are so, so interesting and I would definitely go see that as well. The designer of Black House lives in a house in the back and if you dare you sometimes you can go back and talk to him. They move the sunning anacondas around 5:30ish everyday so we you can even help with that!
Pai. This is a small town about a 3 hr. bus ride north of Chiang Mai. It's known as an organic/hippy/yogi destination that's pretty awesome. It's also a bit off the beaten path and your coworker would be able to do a mediation retreat, a yoga retreat or just relax. Again, it's a small town so she'd have to drive back to Chiang Mai in order to fly back to Bangkok, or anywhere for that matter.