Short stays: weeks to a couple of months
For a quick trip, your options are simple. Visa-free entry for ASEAN passport holders gives 30 days with no extension. The Visa on Arrival gives 30 days and can be extended once for another 30, so about 60 days in total. If you want to come and go, a D212 multiple-entry visa allows up to 60 days per entry and stays valid for one to five years, though each individual visit is still capped.