i second try's suggestion: angkor wat in cambodia. this world heritage site is worth all the trouble if you are going there by road from thailand (no paved roads, but stones and dust and dirt everywhere.)
here's a list u may wanna consider, starting from singapore:
- singapore (2 nights: night safari, sentosa, local clubs, geylang, chinatown, little india, kampong glam)
- malaysia (KL, malacca, cameron highlands, taman negara, redang or perhentian, mt kinabalu)
- thailand (phuket, bangkok, chiangmai, local hilltribes, lots of temples, caves, waterfalls)
- cambodia (siem reap & angkor wat)
- Myanmar esp Bagan, Yangon, and Mandalay (as recommended by bunnychow)
- Vietnam (i'm not sure on the nice spots. perhaps someone can give u more details)
- Indonesia (Bali, Borobudor-check the spelling).
u may not be able to cover all in 20 days so it all depends what you really want to see & experience for your trip. good luck!