You are planning to cover a lot of territory in 20 days. We need more information to be helpful to you.

Where are you flying from? What is your port of entry to Asia? What kinds of things do you enjoy doing ?

We can help you more with accommodations of you give us a per night price. Here are a couple of reliable hotel booking sites to give you an idea:
www.sawasdee.com
www.agoda.com
www.asiarooms.com

Have you purchased guidebooks yet for all of those places? If not, do so soon so you can begin to refine your plans. Do buy a copy of Dawn Rooney's book, Angkor: A Guide to Cambodia's Wonderous Temples

Also, spend a bit of time here on Fodors ready other's trip reports.

Thailand and Bali are great stops. In Thailand, I found Koh Phi Phi and the other islands near Krabi the best for some romantic settings.

I know it doesn't have a beach or anything but I found Chiang Mai in Thailand very charming.

I agree with katie but you do not have to put all your money up front on a reservation with sawadee.com and thatis why I use them. I do find them $3-$6 higher sometimes but I just rather not but up hundreds of dollars 5-6 months before a trip. personally I would spend 6-7 days in bali, 3 days in Chiangi Mai, 2 in Angor Wat, 4 in Bangkok and 4-5 in Krabi or if we want real quiet and no social life then Phi Phi Phi Dom. You may look into a Bankok Air pass for your internal flights if they work in tems of schedule works for you. But to kill 4-5 day time hours waiting for a plane is not worthwhile. This would give you a real feel for many types of like in Thailand. You may be able to stay in a 4 star hotel in Bangkok if you are going in the next 3 months because while their demonstrations are all settled the hotel rates could be $125/night in a 4+ star hotel.

Thanks guys for the replies.

We are flying from Mumbai to Singapore and then flying back from Bangkok to Mumbai. (this could also be reversed as in Mumbai - Bangkok and Singapore - Mumbai).

We are looking at both adventure and peaceful places. Would really like to know what are the good places for both such activities.

Also what is the best itinerary that someone can recommend to take if we want to travel from 3rd Jan to 22nd Jan 2011.

Also is there anything we shouldnt miss for sure. Are there any hotel/flight deals that are available online for these countries as in America/Europe?

Thanks and please let me know.