Impossible for us to decide for you. Every person has different interest and we do not know you. You are in a better position than us to know what you will like.

I find it helps to narrow things down if you have a theme. But my theme wouldn't be yours. You have a wonderful opportunity, but only you know whether you'd rather visit SEA or SA, Cambodia or Laos, etc.

costs can have a lot to do with it... india is huge..

thailand, sri lanka, myanmar, vietnam, bali, malaysia, singapore and many other countries deserve your attention

Good advice from thursdaysd above. It is essential to narrow things down to what you consider the "must sees" in terms of countries and then the individual places within those countries. devising an outline plan based on the above is IMO , essential but build in plenty of time in each country to explore the unexpected.

I assume with that length of trip you will be staying in hostel type accommodation rather than 5* hotel. if so, these are a great place to meet like minded travellers who will, I am sure, have many suggestion to visit places you had never even thought of.

Guide books are great but on this sort of trip probably leave you with more questions than answers. Sites like www.travelfish.org and www.talesofasia.com are better at providing current information for trips like yours.

If you are seeking out cultural experiences such as festival then good timing is essential. Songkran in Thailand and Laos is fun as is Tet in Vietnam.

We did a very similar trip to yours in a few years ago which started off with voluntary work in Sierra Leone and ended with a month in Buenos Aires. In between there were many highlights and we did some more volunteering along the way purely as a result of chance encounters.

We had been to South East Asia many times before and so did not visit many of the places a first timer would as we had seen many of them before. We chose not to visit China but wish we had spent much more time in Japan.

Laos would also be high on my list as would Vietnam but the list is very long and very subjective.

Here is a link to my travel blog with detail of teh route we took, some of the places visited and quite a lot of photos.

http://blog.travelpod.com/travel-blog/candcthai/1/tpod.html

Finally, as you are travelling for 16 months, do not underestimate the effects of "travel fatigue". Build in some places where you can stay for a few weeks just to relax, unpack and recharge your batteries..

You will likely find guidebooks very helpful in deciding where to go, but the websites crellston mentions are great for details of travel arrangements.

Choose the most important places/experiences for you, then build your itinerary around those. And, as others have said, give yourself plenty of time in each place so you can have a real experience of the place.

Also build in time to relax, especially with such a long trip. It might be a few days at a beach or some time up in the mountains where it's cool for a break from the relentless heat.

Check out the fantastic site trufflepig.com. Not only will you get ideas, but they can also help with your trip.

16 months is a lot of time and you are very lucky to have it!

One bit of advice I would give you is "less is more". It is not a race to see as many countries as possible. With this amount of time I would prefer to see 10 countries "properly", i.e. one month +, rather than 25 countries only a little, i.e. 2-3 weeks. Spend 2 months in places like China, Vietnam, Indonesia etc. It will save you so much on costs and give you such a good experience. There is nothing wrong with having days where you just linger in a cafe and do nothing more than read a book surrounded by locals.

Wow Marien22. You are so lucky to have so much time, and more so, gumption to dedicate it to travel, without getting stuck in rat-race! Hats off to you!

I agree with the fellow fodorites that you should focus on absorbing experiences in 8-10 countries at max during these 16 months.

As you would anyway cover quite a lot of India in 3 months of volunteering, you could perhaps extend India stay by a month or two more at the end of volunteering, to cover the places which you could not, during the schedule earlier, or places you would love to go back to once more.

How about spending a month in Nepal, followed by another month in Bhutan? Perhaps, you could then shift base to Sri Lanka for a month or so. Then, fly to Thailand for a 3 month long trip. Indonesia, Cambodia deserve another couple of months for sure. You would certainly enjoy Malaysia and can easily spend couple of months there. Now that you are in Malaysia, you could certainly make a trip to Singapore. While I have lived there for years (owing to work) and I love the city as it provides one of the best standards of living and is quite organised and disciplined, I think you may not cherish your stay there as much as the other Asian countries which thrive more in the chaos and contradictions. Probably, one of those days, when you start missing the comforts of western world and crave for urban touch, Singapore would fit in nicely in between.

I can go and on, but you should first have a handle on what themes / experiences you would like to have. There are a lot of places you could cover and you can shortlist based on your own interests.

Glad to help if you have further specific qs on India and other SE Asian countries.