Thailand is remarkably safe. Yes, like anywhere, you have to keep your eyes open, but crime against travelers is quite rare.

In Bangkok, you'll want to stay somewhere close to a skytrain station or on the river to facilitate getting around without too much interference from the traffic.

I find Chiang Mai harder to get around as there is not public transport. You can catch a tuk tuk or songthew but you may need to hire a car and driver if you want to go see Doi Suthep or do any serious shopping.

Phuket covers a huge area with some beaches that are busy and even sleazy and others that are quiet. Other places on the Andaman Sea include PhiPhi, Krabi, Ko Lanta, Khoa Lak. Krabi has dramatic scenery. You can fly driectly into Krabi. Another beach option is Hua Hin, about a 2 hour drive from Bangkok.

In BKK I recommend the Baan Sukhumvit Inns, one is on Soi 18 and one on Soi 20. Lovely people, great places, very affordable. I stayed in 18 last time I was there in the fall, less expensive than 20 which is the new and more popular as a result. http://www.baansukhumvit.com/

In Chiang Mai I like Gap's House, have stayed elsewhere just once and will go back to Gap's next time. They don't take reservations but show up mid-morning and they almost always have something. Good location, price and crowd. Also cooking school if it interests you. http://www.gaps-house.com/

Sorry, I hit post too soon.

Pick up a good guidebook to help you decide where to go. For accommodations, take a look at a Thai booking site, www.sawasdee.com or other booking sites. You'll get better prices on teh web than you will walking in.

BTW, my first trip to Thailand was 9 full days in Bangkok. I loved it and have been back most years since.

Bangkok is really worth at least several days. In fact, I'd give Bangkok 4, Chiang Mai maybe 3, and then the beach for the remainder of your time. I regularly stay in Bangkok (no side trips out of the city) for two weeks at a time.

Bangkok is safe. I am often there by myself. I visit Bangkok usually 4 times per year (30 trips in all thus far, with #31 coming up in April). I can understand your not wanting to stay in the thick of it all, but some of the smaller sois (side streets) off of Sukhumvit have a neighborhood feel to them. Soi 18 was mentioned, for example. I stay on soi 8, Adelphi Suites, which is but 2 minutes to the escalator UP to the skytrain station at Nana. Being near the skytrain is the #1 objective when in Bangkok!! (and having that escalator is a big plus) Time in taxis, even though they are cheap, is spent sitting in sometimes huge traffic jams! The skytrain whisks you along above all those cars, buses, tuk tuks, trucks, etc.

You don't exactly say what your budget might be, but you might like the Sheraton Four Points on Sukhumvit soi 15, or perhaps the new Sofitel which is right on Sukhumvit, near soi 15.

Thailand is a wonderful destination in S E Asia. Enjoy your trip!

Thanks for your help. I'm budgeting $100USD/day for lodging, food, and entertainment. I based that on what I saw meals and hotels cost in my initial search. Fortunately airfare is not my largest expenses being able to fly standby on many carriers although my airline (American) has no agreements with Bangkok Airways, which I've heard is the easiest way to get to many cities and a Bangkok Airpass is the most economical way to get around. The Sukhumvit hotels look great. I know that I'm planning this all sort of last minute but flying standby, everything can change depending on seat availability. Is this high season? I generally fly on the off season because it's the only way to get on a flight.

The Bangkok Air Discovery Pass is great if you are going to places that are expensive to fly to: Siem Reap, Luang Prabang, Koh Samui. Otherwise, you are best off flying Air Asia, a budget airline, or if AA has agreements with them, fly Thai.

do you have any FF miles for hotels? bkk is a good place to use them with marriott, hilton, hyatt, sheratons on offer.

you might like to stay in the sukhumvit/plenochit areas for just a couple of nts in bkk--sheraton, marriotts, conrad, 4 seasons... in these areas there are tons of good restaurants and shopping..

a fav is marriott sukhumvit park on soi 24 sukhumvit... near a mall, the skytrain is not far.. decent restaurants and affordable.

personally i would skip chiang mai and add time to bkk and a beach..

hua hin is a 3 hr drive s of bkk... saves flt time.. hyatt and intercontinentals there are nice..

Got elephants? Chiang Mai does. Pet the babies, ride the elders?

I think you should see Chiang Mai. I like it better than BKK. Go this trip and decide for yourself. It's a place you can walk all over and I like that aspect of it, more big town than city.

I was in Thailand for three weeks last winter, traveling solo. Loved every minute. I stayed at the Adelphi that simpsonc mentions (thank you) - and thought it fit my needs perfectly - clean modern rooms, nice staff, food nearby including street food right outside the door. The Skytrain is right at the end of the street, and gets you practically every place you want to go on a first trip to Bangkok - Skytrain to riverboat to reclining buddha or out to the weekend market if you time it right. It was also really easy to take the train from the airport and connect to the Skytrain - no taxi needed (just travel fairly light).
I also spent quite a bit of time in Chiang Mai, and am one of the few that loved it there. I like to walk (or run through traffic as needed), and loved the markets, tuk tuks, etc. I stayed just outside the walls of the old city and at another place across the river. Not low budget, but affordable $100 or so, and felt safe and comfortable at the night markets alone at night, or wandering around during the day. Just use reasonable caution.
I am not a shopper in the mall sort of way, so I sought out the smaller shops and food markets in both Bangkok and Chiang Mai and really enjoyed the time doing so - and the Wats are an endless source of wandering enjoyment. You can also take a food tour in Bangkok, or cooking class in Chiang Mai for very little money.
I definitely plan a second trip. It is not a manicured place, so maybe not everyone's cup of tea - it is the people (and the food) that make it such a pleasure to visit!

We enjoyed Bangkok.. and took this really nice bike tour with a Thai company. The guide was a sweetheart.

http://www.fodors.com/community/asia/amazingly-interesting-bike-tour-near-bangkok.cfm

This bike tour sounds amazing. I will definitely try to squeeze it into my tight trip. Ironically I will be working a flight to/from Tokyo and then will turn around and fly back to Japan in order to get to BKK. Is this high season? How hot is it there?