Activities
- Tours: Some organised tours can be great, especially if you're nervous about a new place, if you're travelling solo or if you're on a really tight schedule. However, tours can be ridiculously expensive and the longer trips can be really limiting (you'll have no flexibility to stay in places you love or leave places you don't). Multiple-day treks usually need a guide and they will have a place for you to sleep (in a village, homestay, tent, etc).
- Cooking classes: I really enjoy doing them, especially if they are small groups and less formal. They can vary massively with some being mostly chopping vegetables and not really learning much. They can also be very expensive, but it's worth doing one if you don't know much about the local cuisine as they often take you to a food market and teach you about the ingredients before you start cooking.
- Food tours can be really interesting, especially if you're new to a region, but they can also be confusingly expensive given the cheap price of food.
- Scuba diving: There are some hotspots for diving in SE Asia.
Kao Tao in Thailand is probably the most famous. It's a good place to get a qualification but the quality of the diving isn't the best in the world. 
Indonesia has some amazing diving, especially around Komodo National Park. - Temples: There's a seemingly endless number of temples in Southeast Asia. Even really small towns can have two or three. You will likely get bored of them very quickly after being "templed out". Of course there are exceptions such as
Bagan in Myanmar and 
Angkor Wat in Cambodia. Some temples have a strict dress code so make sure you have something to go over your shoulders and below your knees. Some of the more popular temples cost to get in.
Elephants and tigers
Excuse the rant but please please please don't ride elephants or go to visit tigers in cages. The elephants are 'broken' (tortured until they give up) and the tigers are permanently sedated. Please do some research (a quick search online and you'll find loads of info like this or recent articles like this) and don't add to the problem. I really believe that if most tourists realised the suffering, they'd not want to be part of it. In October 2016, TripAdvisor stopped selling tickets to such places, although they haven't yet gone as far as refusing to list them.
There are some of sanctuaries which rescue elephants and help to educate visitors to the industry. Clearly they do a lot of good but I believe it's worth considering the potential unintentional side-effects of such places. By posing for photos with elephants and posting them on social media, is it actually increasing the demand for captured/tortured elephants? I've been to a sanctuary (Elephant Nature Park near Chiang Mai, Thailand) and had mixed feelings while I was there. It felt like the elephants were still being exploited.
I love elephants and that's why I don't want to ever see them in captivity again, even if they're treated well.
Events & public holidays
Make sure you check for any festivals or public holidays. Firstly there are some that you might want to visit and others you may need to plan around (like Chinese New Year where many businesses and even government will be closed for up to a week - plus accommodation might be hard to book at short notice).
Unlike at home, weekends and public holidays can actually be a bit of a pain when travelling. Cities and attractions can be overcrowded at weekends and restaurants might be closed on Sundays, for example.
Here are a few major festivals:
- Songkran in Thailand: The water festival is all over Thailand but Chiang Mai is the epicenter.
Thaipusam in Malaysia
Tet in Vietnam: Chinese New Year in Vietnam.- Loi Krathong / Yi Peng in Thailand: Spectacular lantern festival.