Most typical Thai food is very flavorful. There is certainly a fare share of spicy dishes but there are also plenty of other flavors, herbal, fragrant, salty and sweet. Obviously, the difficult part is knowing which is which!
Many of the dishes are not so spicy or salty themselves. Instead, restaurant tables come with small bowls of Nam Pla with chopped chilies floating in them. People scoop this and mix it into the food on their plate after they have taken a portion from a shared serving plate. This makes it possible to make the meal more salty and spicy at the same time. It is difficult to make it more spicy without getting too salty which is the problem I had since I like food very hot but not so salty.
The nice thing is that a group can try different dishes since many dishes are meant to be shared. They can order a few, try them and learn which ones they like. Some curries are relatively mild such as the yellow curry which usually includes potatoes (this is completely different from Indian curry of the same color). Green curries are almost always hot and so are red curries.
The milder dishes are usually grilled marinated meats, either skewered or served over rice. Many rice and noodle dishes are also mild in flavor since they are meant to balance strong flavors from curries and herbs.
When all else fail, order spaghetti or pizza. Almost every Thai restaurant anywhere near tourist areas has one or both of these items specifically for picky foreigners. Since my favorite food is Thai, I have not tried these personally but I wouldn't expect either to be fantastic. You actually often see couples in restaurant with one of the two being Thai and eating Thai food and the other with a plate of spaghetti.
In Vietnam food is completely different. Most of it is much milder in flavor with light seasoning. Grilled meats again are quite popular and can also come with rice or noodles, mostly quite plain, or served in a elongated bread as a sandwich. No idea about Cambodia.