I have never talked to anyone who has been to Asia and had a bad time and if you working to a budet then you get a lot for your money.
Arriving in Bangkok you have loads of options. I you are looking to take in the rest of south east Asia and don’t want to back track on yourself whilst seeing the best bits here is a great itinerary:
Bangkok - River Kwai (Kanchanuburi), Ayutthaya - Chiang Mai (jungle trek) Pai - Chiang Rai - to Laos Boarder - Luang Prabang - Vang Vien - Phonsavan (plain of Jars) - bus over to Hanoi - Hill tribes of Sapa - Halong Bay - Bus down Vietnam stopping at usual spots - HCMC - Take a river cruise along the Mekong river into Cambodia - Phonom Penh - Siem Reap (Angkor Wat) - Bangkok - bus and ferry to Ko Tao - Ko Phangan - Ko Samui - feryy back to main land - Khao Sok national park (amamzing!) - Bus down to Krabi - Ko Lanta - Ko Hai - Jump on a trian down into Malaysia to the Perentian islands - Cameron Highlands - Tamen Negara - Kuala Lumpur - Melaka and onto Singapore. Job done!!!
If you follow this or a similar itinerary then you will not be disappointed. Have fun!