We've explored many parts of SE Asia so far and plan on getting to as many countries as we can in the not so distant future. In our travels we've picked up a few tips and want to pass them on to you.
1. Weather - It will be hot and humid or hot, wet and humid. That's SE Asia in a nutshell, dress and plan for that with lightweight clothing and hats, maybe an umbrella. Bring sunscreen, stay hydrated with bottled water (do not drink the water) and lay low on the liquor while out and about touring during the day. If traveling during the rainy season I would suggest touring in the morning to combat the afternoon rainstorms that occur sometimes daily. Also most Temples require covered shoulders/arms and pants or skirts that cover the knees.
2. Pick a couple of countries/city and stick to that plan. Although you will be tempted to jump around because you may not make it back to this side of the world for some time or everything looks close, you really don't want to spend most of your time in airports or bus depots. I'm a real hard core fan of staying in one city for a week or so seeing everything and anything.
3. Visa- Check the requirements and cost for each country you are contemplating visiting and add those fees to your vacation expenses. Most countries depending on your passport grant easy access and are free but there are exceptions such as highly visited Thailand and Vietnam.
4. Exchange your money once you enter the country or use ATM's, remember ATM safety. Ensure that your card doesn't have major overseas transaction fees or get a card that will reimburse the fees. Remember to alert your bank that you are traveling otherwise they will more than likely cancel it and there is nothing worse than traveling with no credit or debit card with no access to funds.
5. Leave your valuables at home but bring extra power adapters. I'm honestly baffled by folks who go on vacation with thousands of dollars of jewelry. You are a walking target. Sorry but that's a fact.
6. Eat all the food, street and otherwise. But prep before hand with probiotics or some natural or over the counter remedy that you prefer. Don't drink the water and be leary of ice, salads and open fruit. You will more than likely experience some discomfort, expect it as a reward for good eating! Also if you want shots, they are cheaper in country than in the U.S.
7. Travel Insurance. I've feared for my life a few to many times while standing at a corner in Vietnam or riding in the back of a Tuk Tuk in Cambodia. Accidents happen, travel insurance covers you and them.
8. Internet and cell phones. Wifi is everywhere but you will always need it more when you can't access it so I suggest getting a phone that allows you to switch sim cards or getting one of those buy here/pay her cell phones and buying a cheap sim card once you arrive.
9. I believe in bargaining but I don't like haggling. There's a difference. Learn it and be a good tourist and contribute to the economy. That extra fifty cents is just that, fifty cents. On the other side of that I don't believe in giving to begging children because it contributes to a system that pulls kids out of school/trade to beg. Instead buy a meal or contribute to an orphanage or school. Don't take pictures of kids with you in the middle and post on social media like you are some great benefactor unless your name is on the schools door.
10. Have fun. Explore, go outside of comfort zone but do it ethically.